Error in user-mode calculation of ELF program's brk

[Langston]

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.

[Thomas Huth (th-huth)]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" within the next 60 days (otherwise it will get
closed as "Expired"). We will then eventually migrate the ticket auto-
matically to the new system (but you won't be the reporter of the bug
in the new system and thus won't get notified on changes anymore).

Thank you and sorry for the inconvenience.

[Thomas Huth (th-huth)]

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