/* On-demand PLT fixup for shared objects.
- Copyright (C) 1995-2016 Free Software Foundation, Inc.
+ Copyright (C) 1995-2019 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
const PLTREL *const reloc
= (const void *) (D_PTR (l, l_info[DT_JMPREL]) + reloc_offset);
const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
+ const ElfW(Sym) *refsym = sym;
void *const rel_addr = (void *)(l->l_addr + reloc->r_offset);
lookup_t result;
DL_FIXUP_VALUE_TYPE value;
of the object that defines sym. Now add in the symbol
offset. */
value = DL_FIXUP_MAKE_VALUE (result,
- sym ? (LOOKUP_VALUE_ADDRESS (result)
- + sym->st_value) : 0);
+ SYMBOL_ADDRESS (result, sym, false));
}
else
{
/* We already found the symbol. The module (and therefore its load
address) is also known. */
- value = DL_FIXUP_MAKE_VALUE (l, l->l_addr + sym->st_value);
+ value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, sym, true));
result = l;
}
if (__glibc_unlikely (GLRO(dl_bind_not)))
return value;
- return elf_machine_fixup_plt (l, result, reloc, rel_addr, value);
+ return elf_machine_fixup_plt (l, result, refsym, sym, reloc, rel_addr, value);
}
#ifndef PROF
/* This is the address in the array where we store the result of previous
relocations. */
struct reloc_result *reloc_result = &l->l_reloc_result[reloc_index];
- DL_FIXUP_VALUE_TYPE *resultp = &reloc_result->addr;
- DL_FIXUP_VALUE_TYPE value = *resultp;
- if (DL_FIXUP_VALUE_CODE_ADDR (value) == 0)
+ /* CONCURRENCY NOTES:
+
+ Multiple threads may be calling the same PLT sequence and with
+ LD_AUDIT enabled they will be calling into _dl_profile_fixup to
+ update the reloc_result with the result of the lazy resolution.
+ The reloc_result guard variable is reloc_init, and we use
+ acquire/release loads and store to it to ensure that the results of
+ the structure are consistent with the loaded value of the guard.
+ This does not fix all of the data races that occur when two or more
+ threads read reloc_result->reloc_init with a value of zero and read
+ and write to that reloc_result concurrently. The expectation is
+ generally that while this is a data race it works because the
+ threads write the same values. Until the data races are fixed
+ there is a potential for problems to arise from these data races.
+ The reloc result updates should happen in parallel but there should
+ be an atomic RMW which does the final update to the real result
+ entry (see bug 23790).
+
+ The following code uses reloc_result->init set to 0 to indicate if it is
+ the first time this object is being relocated, otherwise 1 which
+ indicates the object has already been relocated.
+
+ Reading/Writing from/to reloc_result->reloc_init must not happen
+ before previous writes to reloc_result complete as they could
+ end-up with an incomplete struct. */
+ DL_FIXUP_VALUE_TYPE value;
+ unsigned int init = atomic_load_acquire (&reloc_result->init);
+
+ if (init == 0)
{
/* This is the first time we have to relocate this object. */
const ElfW(Sym) *const symtab
of the object that defines sym. Now add in the symbol
offset. */
value = DL_FIXUP_MAKE_VALUE (result,
- defsym != NULL
- ? LOOKUP_VALUE_ADDRESS (result)
- + defsym->st_value : 0);
+ SYMBOL_ADDRESS (result, defsym, false));
if (defsym != NULL
&& __builtin_expect (ELFW(ST_TYPE) (defsym->st_info)
{
/* We already found the symbol. The module (and therefore its load
address) is also known. */
- value = DL_FIXUP_MAKE_VALUE (l, l->l_addr + refsym->st_value);
+ value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, refsym, true));
if (__builtin_expect (ELFW(ST_TYPE) (refsym->st_info)
== STT_GNU_IFUNC, 0))
/* Store the result for later runs. */
if (__glibc_likely (! GLRO(dl_bind_not)))
- *resultp = value;
+ {
+ reloc_result->addr = value;
+ /* Guarantee all previous writes complete before
+ init is updated. See CONCURRENCY NOTES earlier */
+ atomic_store_release (&reloc_result->init, 1);
+ }
+ init = 1;
}
+ else
+ value = reloc_result->addr;
/* By default we do not call the pltexit function. */
long int framesize = -1;
+
#ifdef SHARED
/* Auditing checkpoint: report the PLT entering and allow the
auditors to change the value. */
- if (DL_FIXUP_VALUE_CODE_ADDR (value) != 0 && GLRO(dl_naudit) > 0
+ if (GLRO(dl_naudit) > 0
/* Don't do anything if no auditor wants to intercept this call. */
&& (reloc_result->enterexit & LA_SYMB_NOPLTENTER) == 0)
{
+ /* Sanity check: DL_FIXUP_VALUE_CODE_ADDR (value) should have been
+ initialized earlier in this function or in another thread. */
+ assert (DL_FIXUP_VALUE_CODE_ADDR (value) != 0);
ElfW(Sym) *defsym = ((ElfW(Sym) *) D_PTR (reloc_result->bound,
l_info[DT_SYMTAB])
+ reloc_result->boundndx);
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