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[Bug 1886097] Re: Error in user-mode calculation of ELF program's brk
From: |
Thomas Huth |
Subject: |
[Bug 1886097] Re: Error in user-mode calculation of ELF program's brk |
Date: |
Wed, 12 May 2021 11:02:26 -0000 |
This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'expired' now.
Please continue with the discussion here:
https://gitlab.com/qemu-project/qemu/-/issues/276
** Changed in: qemu
Status: New => Expired
** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #276
https://gitlab.com/qemu-project/qemu/-/issues/276
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https://bugs.launchpad.net/bugs/1886097
Title:
Error in user-mode calculation of ELF program's brk
Status in QEMU:
Expired
Bug description:
There's a discrepancy between the way QEMU user-mode and Linux
calculate the initial program break for statically-linked binaries. I
have a binary with the following segments:
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
EXIDX 0x065a14 0x00075a14 0x00075a14 0x00588 0x00588 R 0x4
PHDR 0x0a3000 0x000a3000 0x000a3000 0x00160 0x00160 R 0x1000
LOAD 0x0a3000 0x000a3000 0x000a3000 0x00160 0x00160 R 0x1000
LOAD 0x000000 0x00010000 0x00010000 0x65fa0 0x65fa0 R E 0x10000
LOAD 0x066b7c 0x00086b7c 0x00086b7c 0x02384 0x02384 RW 0x10000
NOTE 0x000114 0x00010114 0x00010114 0x00044 0x00044 R 0x4
TLS 0x066b7c 0x00086b7c 0x00086b7c 0x00010 0x00030 R 0x4
GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0x8
GNU_RELRO 0x066b7c 0x00086b7c 0x00086b7c 0x00484 0x00484 R 0x1
LOAD 0x07e000 0x00089000 0x00089000 0x03ff4 0x03ff4 R E 0x1000
LOAD 0x098000 0x00030000 0x00030000 0x01000 0x01000 RW 0x1000
The call to set_brk in Linux's binfmt_elf.c receives these arguments:
set_brk(0xa3160, 0xa3160, 1)
Whereas in QEMU, info->brk gets set to 0x88f00. When the binary is run in
QEMU, it crashes on the second call to brk, whereas it runs fine on real ARM
hardware. I think the trouble is that the program break is set to an address
lower than the virtual address of a LOAD segment (the program headers, in this
case).
I believe that this discrepancy arises because in QEMU, info->brk is
only incremented when the LOAD segment in question has PROT_WRITE. For
this binary, the LOAD segment with write permissions and the highest
virtual address is
LOAD 0x066b7c 0x00086b7c 0x00086b7c 0x02384 0x02384 RW 0x10000
which overlaps with the TLS segment:
TLS 0x066b7c 0x00086b7c 0x00086b7c 0x00010 0x00030 R 0x4
However, the Linux kernel puts the program break after the loadable segment
with the highest virtual address, regardless of flags. So I think the fix is
for QEMU to do the same.
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