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From 5ccae46f2a192a9347feb604901127c55ce1e039 Mon Sep 17 00:00:00 2001
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From: Kyle Walker <kwalker@redhat.com>
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Date: Wed, 13 Dec 2017 12:49:26 -0500
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Subject: [PATCH] core: Implement timeout based umount/remount limit
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Remount, and subsequent umount, attempts can hang for inaccessible network
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based mount points. This can leave a system in a hard hang state that
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requires a hard reset in order to recover. This change moves the remount,
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and umount attempts into separate child processes. The remount and umount
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operations will block for up to 90 seconds (DEFAULT_TIMEOUT_USEC). Should
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those waits fail, the parent will issue a SIGKILL to the child and continue
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with the shutdown efforts.
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In addition, instead of only reporting some additional errors on the final
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attempt, failures are reported as they occur.
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(cherry picked from commit d5641e0d7e8f55937fbc3a7ecd667e42c5836d80)
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Related: #1571098
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---
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 src/core/umount.c         | 112 ++++++++++++++++++++++++++++++--------
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 src/shared/def.h          |   2 -
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 src/shared/login-shared.c |   1 +
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 src/shared/util.c         |  61 +++++++++++++++++++++
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 src/shared/util.h         |  16 ++++++
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 5 files changed, 168 insertions(+), 24 deletions(-)
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diff --git a/src/core/umount.c b/src/core/umount.c
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index 91d67c06ca..bd38966124 100644
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--- a/src/core/umount.c
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+++ b/src/core/umount.c
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@@ -363,7 +363,84 @@ static int delete_dm(dev_t devnum) {
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         return r >= 0 ? 0 : -errno;
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 }
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-static int mount_points_list_umount(MountPoint **head, bool *changed, bool log_error) {
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+static int remount_with_timeout(MountPoint *m, char *options, int *n_failed) {
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+        pid_t pid;
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+        int r;
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+
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+        BLOCK_SIGNALS(SIGCHLD);
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+
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+        /* Due to the possiblity of a remount operation hanging, we
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+         * fork a child process and set a timeout. If the timeout
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+         * lapses, the assumption is that that particular remount
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+         * failed. */
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+        pid = fork();
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+        if (pid < 0)
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+                return log_error_errno(errno, "Failed to fork: %m");
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+
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+        if (pid == 0) {
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+                log_info("Remounting '%s' read-only in with options '%s'.", m->path, options);
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+
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+                /* Start the mount operation here in the child */
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+                r = mount(NULL, m->path, NULL, MS_REMOUNT|MS_RDONLY, options);
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+                if (r < 0)
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+                        log_error_errno(errno, "Failed to remount '%s' read-only: %m", m->path);
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+
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+                _exit(r < 0 ? EXIT_FAILURE : EXIT_SUCCESS);
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+        }
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+
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+        r = wait_for_terminate_with_timeout(pid, DEFAULT_TIMEOUT_USEC);
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+        if (r == -ETIMEDOUT) {
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+                log_error_errno(errno, "Remounting '%s' - timed out, issuing SIGKILL to PID "PID_FMT".", m->path, pid);
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+                (void) kill(pid, SIGKILL);
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+        } else if (r < 0)
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+                log_error_errno(r, "Failed to wait for process: %m");
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+
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+        return r;
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+}
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+
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+static int umount_with_timeout(MountPoint *m, bool *changed) {
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+        pid_t pid;
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+        int r;
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+
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+        BLOCK_SIGNALS(SIGCHLD);
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+
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+        /* Due to the possiblity of a umount operation hanging, we
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+         * fork a child process and set a timeout. If the timeout
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+         * lapses, the assumption is that that particular umount
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+         * failed. */
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+        pid = fork();
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+        if (pid < 0)
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+                return log_error_errno(errno, "Failed to fork: %m");
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+
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+        if (pid == 0) {
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+                log_info("Unmounting '%s'.", m->path);
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+
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+                /* Start the mount operation here in the child Using MNT_FORCE
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+                 * causes some filesystems (e.g. FUSE and NFS and other network
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+                 * filesystems) to abort any pending requests and return -EIO
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+                 * rather than blocking indefinitely. If the filesysten is
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+                 * "busy", this may allow processes to die, thus making the
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+                 * filesystem less busy so the unmount might succeed (rather
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+                 * then return EBUSY).*/
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+                r = umount2(m->path, MNT_FORCE);
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+                if (r < 0)
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+                        log_error_errno(errno, "Failed to unmount %s: %m", m->path);
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+
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+                _exit(r < 0 ? EXIT_FAILURE : EXIT_SUCCESS);
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+        }
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+
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+        r = wait_for_terminate_with_timeout(pid, DEFAULT_TIMEOUT_USEC);
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+        if (r == -ETIMEDOUT) {
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+                log_error_errno(errno, "Unmounting '%s' - timed out, issuing SIGKILL to PID "PID_FMT".", m->path, pid);
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+                (void) kill(pid, SIGKILL);
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+        } else if (r < 0)
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+                log_error_errno(r, "Failed to wait for process: %m");
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+
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+        return r;
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+}
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+
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+
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+static int mount_points_list_umount(MountPoint **head, bool *changed) {
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         MountPoint *m, *n;
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         int n_failed = 0;
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@@ -405,9 +482,13 @@ static int mount_points_list_umount(MountPoint **head, bool *changed, bool log_e
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                          * explicitly remount the super block of that
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                          * alias read-only we hence should be
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                          * relatively safe regarding keeping the fs we
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-                         * can otherwise not see dirty. */
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-                        log_info("Remounting '%s' read-only with options '%s'.", m->path, options);
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-                        (void) mount(NULL, m->path, NULL, MS_REMOUNT|MS_RDONLY, options);
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+                         * can otherwise not see dirty.
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+                         *
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+                         * Since the remount can hang in the instance of
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+                         * remote filesystems, we remount asynchronously
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+                         * and skip the subsequent umount if it fails */
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+                        if (remount_with_timeout(m, options, &n_failed) < 0)
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+                                continue;
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                 }
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                 /* Skip / and /usr since we cannot unmount that
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@@ -420,22 +501,14 @@ static int mount_points_list_umount(MountPoint **head, bool *changed, bool log_e
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                 )
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                         continue;
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-                /* Trying to umount. Using MNT_FORCE causes some
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-                 * filesystems (e.g. FUSE and NFS and other network
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-                 * filesystems) to abort any pending requests and
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-                 * return -EIO rather than blocking indefinitely.
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-                 * If the filesysten is "busy", this may allow processes
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-                 * to die, thus making the filesystem less busy so
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-                 * the unmount might succeed (rather then return EBUSY).*/
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-                log_info("Unmounting %s.", m->path);
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-                if (umount2(m->path, MNT_FORCE) == 0) {
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+                /* Trying to umount */
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+                if (umount_with_timeout(m, changed) < 0)
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+                        n_failed++;
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+                else {
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                         if (changed)
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                                 *changed = true;
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                         mount_point_free(head, m);
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-                } else if (log_error) {
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-                        log_warning_errno(errno, "Could not unmount %s: %m", m->path);
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-                        n_failed++;
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                 }
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         }
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@@ -550,17 +623,12 @@ int umount_all(bool *changed) {
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         do {
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                 umount_changed = false;
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-                mount_points_list_umount(&mp_list_head, &umount_changed, false);
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+                mount_points_list_umount(&mp_list_head, &umount_changed);
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                 if (umount_changed)
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                         *changed = true;
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         } while (umount_changed);
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-        /* umount one more time with logging enabled */
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-        r = mount_points_list_umount(&mp_list_head, &umount_changed, true);
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-        if (r <= 0)
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-                goto end;
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-
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   end:
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         mount_points_list_free(&mp_list_head);
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diff --git a/src/shared/def.h b/src/shared/def.h
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index 9e008a6d2d..f193ab1f9f 100644
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--- a/src/shared/def.h
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+++ b/src/shared/def.h
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@@ -21,8 +21,6 @@
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   along with systemd; If not, see <http://www.gnu.org/licenses/>.
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 ***/
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-#include "util.h"
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-
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 #define DEFAULT_TIMEOUT_USEC (90*USEC_PER_SEC)
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 #define DEFAULT_RESTART_USEC (100*USEC_PER_MSEC)
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 #define DEFAULT_CONFIRM_USEC (30*USEC_PER_SEC)
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diff --git a/src/shared/login-shared.c b/src/shared/login-shared.c
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index 054c77503b..5da0f05838 100644
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--- a/src/shared/login-shared.c
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+++ b/src/shared/login-shared.c
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@@ -21,6 +21,7 @@
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 #include "login-shared.h"
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 #include "def.h"
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+#include "util.h"
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 bool session_id_valid(const char *id) {
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         assert(id);
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diff --git a/src/shared/util.c b/src/shared/util.c
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index 982f5e044f..3216f004ad 100644
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--- a/src/shared/util.c
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+++ b/src/shared/util.c
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@@ -9049,3 +9049,64 @@ try_dev_shm_without_o_tmpfile:
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         return -EOPNOTSUPP;
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 }
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+
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+/*
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+ * Return values:
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+ * < 0 : wait_for_terminate_with_timeout() failed to get the state of the
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+ *       process, the process timed out, the process was terminated by a
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+ *       signal, or failed for an unknown reason.
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+ * >=0 : The process terminated normally with no failures.
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+ *
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+ * Success is indicated by a return value of zero, a timeout is indicated
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+ * by ETIMEDOUT, and all other child failure states are indicated by error
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+ * is indicated by a non-zero value.
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+*/
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+int wait_for_terminate_with_timeout(pid_t pid, usec_t timeout) {
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+        sigset_t mask;
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+        int r;
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+        usec_t until;
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+
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+        assert_se(sigemptyset(&mask) == 0);
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+        assert_se(sigaddset(&mask, SIGCHLD) == 0);
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+
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+        /* Drop into a sigtimewait-based timeout. Waiting for the
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+         * pid to exit. */
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+        until = now(CLOCK_MONOTONIC) + timeout;
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+        for (;;) {
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+                usec_t n;
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+                siginfo_t status = {};
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+                struct timespec ts;
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+
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+                n = now(CLOCK_MONOTONIC);
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+                if (n >= until)
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+                        break;
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+
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+                r = sigtimedwait(&mask, NULL, timespec_store(&ts, until - n)) < 0 ? -errno : 0;
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+                /* Assuming we woke due to the child exiting. */
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+                if (waitid(P_PID, pid, &status, WEXITED|WNOHANG) == 0) {
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+                        if (status.si_pid == pid) {
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+                                /* This is the correct child.*/
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+                                if (status.si_code == CLD_EXITED)
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+                                        return (status.si_status == 0) ? 0 : -EPROTO;
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+                                else
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+                                        return -EPROTO;
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+                        }
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+                }
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+                /* Not the child, check for errors and proceed appropriately */
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+                if (r < 0) {
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+                        switch (r) {
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+                        case -EAGAIN:
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+                                /* Timed out, child is likely hung. */
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+                                return -ETIMEDOUT;
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+                        case -EINTR:
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+                                /* Received a different signal and should retry */
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+                                continue;
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+                        default:
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+                                /* Return any unexpected errors */
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+                                return r;
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+                        }
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+                }
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+        }
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+
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+        return -EPROTO;
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+}
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diff --git a/src/shared/util.h b/src/shared/util.h
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index 9c4be02566..998f882bbb 100644
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--- a/src/shared/util.h
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+++ b/src/shared/util.h
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@@ -22,6 +22,7 @@
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 ***/
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 #include <alloca.h>
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+#include <def.h>
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 #include <fcntl.h>
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 #include <inttypes.h>
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 #include <time.h>
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@@ -1122,3 +1123,18 @@ enum {
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 };
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 int acquire_data_fd(const void *data, size_t size, unsigned flags);
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+
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+int wait_for_terminate_with_timeout(pid_t pid, usec_t timeout);
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+
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+static inline void block_signals_reset(sigset_t *ss) {
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+        assert_se(sigprocmask(SIG_SETMASK, ss, NULL) >= 0);
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+}
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+
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+#define BLOCK_SIGNALS(...)                                                         \
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+        _cleanup_(block_signals_reset) _unused_ sigset_t _saved_sigset = ({        \
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+                sigset_t _t;                                                       \
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+                assert_se(sigprocmask(SIG_SETMASK, NULL, &_t) == 0);               \
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+                assert_se(sigprocmask_many(SIG_BLOCK, __VA_ARGS__, -1) >= 0);      \
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+                _t;                                                                \
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+        })
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+