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commit 4a2ab5843a5cc4a5db1b3b79916a520ea8b115dc
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Author: Florian Weimer <fweimer@redhat.com>
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Date: Fri Nov 8 15:48:51 2019 +0100
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dlsym: Do not determine caller link map if not needed
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Obtaining the link map is potentially very slow because it requires
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iterating over all loaded objects in the current implementation. If
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the caller supplied an explicit handle (i.e., not one of the RTLD_*
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constants), the dlsym implementation does not need the identity of the
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caller (except in the special case of auditing), so this change
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avoids computing it in that case.
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Even in the minimal case (dlsym called from a main program linked with
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-dl), this shows a small speedup, perhaps around five percent. The
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performance improvement can be arbitrarily large in principle (if
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_dl_find_dso_for_object has to iterate over many link maps).
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Change-Id: Ide5d9e2cc7ac25a0ffae8fb4c26def0c898efa29
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diff --git a/elf/dl-sym.c b/elf/dl-sym.c
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index 286cf7e27fd59f20..b133850a3c6657a4 100644
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--- a/elf/dl-sym.c
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+++ b/elf/dl-sym.c
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@@ -80,6 +80,18 @@ call_dl_lookup (void *ptr)
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args->flags, NULL);
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}
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+/* Return the link map containing the caller address. */
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+static inline struct link_map *
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+find_caller_link_map (ElfW(Addr) caller)
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+{
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+ struct link_map *l = _dl_find_dso_for_object (caller);
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+ if (l != NULL)
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+ return l;
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+ else
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+ /* If the address is not recognized the call comes from the main
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+ program (we hope). */
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+ return GL(dl_ns)[LM_ID_BASE]._ns_loaded;
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+}
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static void *
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do_sym (void *handle, const char *name, void *who,
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@@ -89,13 +101,13 @@ do_sym (void *handle, const char *name, void *who,
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lookup_t result;
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ElfW(Addr) caller = (ElfW(Addr)) who;
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- struct link_map *l = _dl_find_dso_for_object (caller);
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- /* If the address is not recognized the call comes from the main
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- program (we hope). */
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- struct link_map *match = l ? l : GL(dl_ns)[LM_ID_BASE]._ns_loaded;
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+ /* Link map of the caller if needed. */
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+ struct link_map *match = NULL;
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if (handle == RTLD_DEFAULT)
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{
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+ match = find_caller_link_map (caller);
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+
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/* Search the global scope. We have the simple case where
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we look up in the scope of an object which was part of
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the initial binary. And then the more complex part
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@@ -128,6 +140,8 @@ do_sym (void *handle, const char *name, void *who,
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}
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else if (handle == RTLD_NEXT)
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{
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+ match = find_caller_link_map (caller);
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+
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if (__glibc_unlikely (match == GL(dl_ns)[LM_ID_BASE]._ns_loaded))
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{
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if (match == NULL
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@@ -187,6 +201,9 @@ RTLD_NEXT used in code not dynamically loaded"));
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unsigned int ndx = (ref - (ElfW(Sym) *) D_PTR (result,
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l_info[DT_SYMTAB]));
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+ if (match == NULL)
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+ match = find_caller_link_map (caller);
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+
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if ((match->l_audit_any_plt | result->l_audit_any_plt) != 0)
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{
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unsigned int altvalue = 0;
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