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linux-user: Fix loading of BSS segments 

Some ELF binaries encode the .bss section as an extension of the data
ones by setting the segment p_memsz > p_filesz. Some other binaries take
a different route and encode it as a stand-alone PT_LOAD segment with
p_filesz = 0 and p_memsz > 0.

Both the encodings are actually correct per ELF specification but the
ELF loader had some troubles in handling the former: with the old logic
it was very likely to get Qemu to crash in zero_bss when trying to
access unmapped memory.

zero_bss isn't meant to allocate whole zero-filled segments but to
"complete" a previously mapped segment with the needed zero bits.

The fix is pretty simple, if the segment is completely zero-filled we
simply allocate one or more pages (according to p_memsz) and avoid
calling zero_bss altogether.

Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
Message-Id: <c9106487-dc4d-120a-bd48-665b3c617287@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
This commit is contained in:
Giuseppe Musacchio 2020-12-17 11:17:40 +01:00 committed by Laurent Vivier
parent 7872130113
commit 22d113b52f
1 changed files with 20 additions and 10 deletions
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30
linux-user/elfload.c
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@ -2805,14 +2805,16 @@ static void load_elf_image(const char *image_name, int image_fd,
vaddr = load_bias + eppnt->p_vaddr;
vaddr_po = TARGET_ELF_PAGEOFFSET(vaddr);
vaddr_ps = TARGET_ELF_PAGESTART(vaddr);
vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + vaddr_po);
vaddr_ef = vaddr + eppnt->p_filesz;
vaddr_em = vaddr + eppnt->p_memsz;
/*
* Some segments may be completely empty without any backing file
* segment, in that case just let zero_bss allocate an empty buffer
* for it.
* Some segments may be completely empty, with a non-zero p_memsz
* but no backing file segment.
*/
if (eppnt->p_filesz != 0) {
vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_filesz + vaddr_po);
error = target_mmap(vaddr_ps, vaddr_len, elf_prot,
MAP_PRIVATE | MAP_FIXED,
image_fd, eppnt->p_offset - vaddr_po);
@ -2820,14 +2822,22 @@ static void load_elf_image(const char *image_name, int image_fd,
if (error == -1) {
goto exit_mmap;
}
}
vaddr_ef = vaddr + eppnt->p_filesz;
vaddr_em = vaddr + eppnt->p_memsz;
/*
* If the load segment requests extra zeros (e.g. bss), map it.
*/
if (eppnt->p_filesz < eppnt->p_memsz) {
zero_bss(vaddr_ef, vaddr_em, elf_prot);
}
} else if (eppnt->p_memsz != 0) {
vaddr_len = TARGET_ELF_PAGELENGTH(eppnt->p_memsz + vaddr_po);
error = target_mmap(vaddr_ps, vaddr_len, elf_prot,
MAP_PRIVATE | MAP_FIXED | MAP_ANONYMOUS,
-1, 0);
/* If the load segment requests extra zeros (e.g. bss), map it. */
if (vaddr_ef < vaddr_em) {
zero_bss(vaddr_ef, vaddr_em, elf_prot);
if (error == -1) {
goto exit_mmap;
}
}
/* Find the full program boundaries. */