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

[Langston]

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

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

Status in QEMU:
  New

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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