[Bug 1886097] Re: Error in user-mode calculation of ELF program's brk

[Thomas 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.

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Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

** Tags added: linux-user

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https://bugs.launchpad.net/bugs/1886097

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

Status in QEMU:
  Incomplete

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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