Blame SOURCES/kvm-block-file-posix-File-locking-during-creation.patch

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From f815f89794a322fd3d0c7275ecdebce5a50e4307 Mon Sep 17 00:00:00 2001
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From: Max Reitz <mreitz@redhat.com>
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Date: Mon, 18 Jun 2018 14:53:36 +0200
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Subject: [PATCH 155/268] block/file-posix: File locking during creation
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RH-Author: Max Reitz <mreitz@redhat.com>
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Message-id: <20180618145337.633-3-mreitz@redhat.com>
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Patchwork-id: 80749
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O-Subject: [RHV-7.6 qemu-kvm-rhev PATCH 2/3] block/file-posix: File locking during creation
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Bugzilla: 1519144
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
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When creating a file, we should take the WRITE and RESIZE permissions.
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We do not need either for the creation itself, but we do need them for
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clearing and resizing it.  So we can take the proper permissions by
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replacing O_TRUNC with an explicit truncation to 0, and by taking the
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appropriate file locks between those two steps.
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Signed-off-by: Max Reitz <mreitz@redhat.com>
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Message-id: 20180509215336.31304-3-mreitz@redhat.com
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Reviewed-by: Fam Zheng <famz@redhat.com>
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Signed-off-by: Max Reitz <mreitz@redhat.com>
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(cherry picked from commit b8cf1913a989b9ea6f248aaa233330047a62962e)
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Signed-off-by: Max Reitz <mreitz@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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 block/file-posix.c | 37 +++++++++++++++++++++++++++++++++++--
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 1 file changed, 35 insertions(+), 2 deletions(-)
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diff --git a/block/file-posix.c b/block/file-posix.c
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index c98a4a1..370a483 100644
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--- a/block/file-posix.c
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+++ b/block/file-posix.c
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@@ -1992,6 +1992,7 @@ static int raw_co_create(BlockdevCreateOptions *options, Error **errp)
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 {
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     BlockdevCreateOptionsFile *file_opts;
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     int fd;
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+    int perm, shared;
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     int result = 0;
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     /* Validate options and set default values */
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@@ -2006,14 +2007,44 @@ static int raw_co_create(BlockdevCreateOptions *options, Error **errp)
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     }
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     /* Create file */
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-    fd = qemu_open(file_opts->filename, O_RDWR | O_CREAT | O_TRUNC | O_BINARY,
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-                   0644);
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+    fd = qemu_open(file_opts->filename, O_RDWR | O_CREAT | O_BINARY, 0644);
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     if (fd < 0) {
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         result = -errno;
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         error_setg_errno(errp, -result, "Could not create file");
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         goto out;
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     }
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+    /* Take permissions: We want to discard everything, so we need
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+     * BLK_PERM_WRITE; and truncation to the desired size requires
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+     * BLK_PERM_RESIZE.
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+     * On the other hand, we cannot share the RESIZE permission
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+     * because we promise that after this function, the file has the
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+     * size given in the options.  If someone else were to resize it
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+     * concurrently, we could not guarantee that.
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+     * Note that after this function, we can no longer guarantee that
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+     * the file is not touched by a third party, so it may be resized
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+     * then. */
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+    perm = BLK_PERM_WRITE | BLK_PERM_RESIZE;
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+    shared = BLK_PERM_ALL & ~BLK_PERM_RESIZE;
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+
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+    /* Step one: Take locks */
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+    result = raw_apply_lock_bytes(fd, perm, shared, false, errp);
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+    if (result < 0) {
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+        goto out_close;
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+    }
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+
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+    /* Step two: Check that nobody else has taken conflicting locks */
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+    result = raw_check_lock_bytes(fd, perm, shared, errp);
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+    if (result < 0) {
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+        goto out_close;
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+    }
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+
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+    /* Clear the file by truncating it to 0 */
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+    result = raw_regular_truncate(fd, 0, PREALLOC_MODE_OFF, errp);
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+    if (result < 0) {
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+        goto out_close;
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+    }
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+
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     if (file_opts->nocow) {
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 #ifdef __linux__
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         /* Set NOCOW flag to solve performance issue on fs like btrfs.
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@@ -2029,6 +2060,8 @@ static int raw_co_create(BlockdevCreateOptions *options, Error **errp)
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 #endif
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     }
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+    /* Resize and potentially preallocate the file to the desired
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+     * final size */
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     result = raw_regular_truncate(fd, file_opts->size, file_opts->preallocation,
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                                   errp);
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     if (result < 0) {
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-- 
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1.8.3.1
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