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elf: Set l_contiguous to 1 for the main map in more cases

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l_contiguous was not initialized at all for the main map and
always 0.  This commit adds code to check if the LOAD segments
are adjacent to each other, and sets l_contiguous accordingly.
This helps _dl_find_object because it is more efficient if the
main mapping is contiguous.

Note that not all (PIE or non-PIE) binaries are contiguous in this
way because BFD ld creates executables with LOAD holes:

ELF LOAD segments creating holes in the process image on GNU/Linux
https://sourceware.org/pipermail/binutils/2022-January/119082.html
https://sourceware.org/bugzilla/show_bug.cgi?id=28743

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
This commit is contained in:
Florian Weimer 2022-01-17 09:57:19 +01:00
parent b4d4ff8963
commit 8eb2510d38
1 changed files with 25 additions and 0 deletions
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25
elf/rtld.c
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@ -1147,6 +1147,22 @@ rtld_setup_main_map (struct link_map *main_map)
main_map->l_map_start = ~0;
/* And it was opened directly. */
++main_map->l_direct_opencount;
main_map->l_contiguous = 1;
/* A PT_LOAD segment at an unexpected address will clear the
l_contiguous flag. The ELF specification says that PT_LOAD
segments need to be sorted in in increasing order, but perhaps
not all executables follow this requirement. Having l_contiguous
equal to 1 is just an optimization, so the code below does not
try to sort the segments in case they are unordered.
There is one corner case in which l_contiguous is not set to 1,
but where it could be set: If a PIE (ET_DYN) binary is loaded by
glibc itself (not the kernel), it is always contiguous due to the
way the glibc loader works. However, the kernel loader may still
create holes in this case, and the code here still uses 0
conservatively for the glibc-loaded case, too. */
ElfW(Addr) expected_load_address = 0;
/* Scan the program header table for the dynamic section. */
for (const ElfW(Phdr) *ph = phdr; ph < &phdr[phnum]; ++ph)
@ -1210,12 +1226,21 @@ rtld_setup_main_map (struct link_map *main_map)
if (main_map->l_map_start > mapstart)
main_map->l_map_start = mapstart;
if (main_map->l_contiguous && expected_load_address != 0
&& expected_load_address != mapstart)
main_map->l_contiguous = 0;
/* Also where it ends. */
allocend = main_map->l_addr + ph->p_vaddr + ph->p_memsz;
if (main_map->l_map_end < allocend)
main_map->l_map_end = allocend;
if ((ph->p_flags & PF_X) && allocend > main_map->l_text_end)
main_map->l_text_end = allocend;
/* The next expected address is the page following this load
segment. */
expected_load_address = ((allocend + GLRO(dl_pagesize) - 1)
& ~(GLRO(dl_pagesize) - 1));
}
break;