[Qemu-devel] [Bug 1462640] Re: shmat fails on 32-to-64 setup

[Ari Sundholm]

I can confirm that this bug still exists in the current qemu master
(short commit ID 0050f9978e):

~/qemu$ gcc -m32 shm_bug.c -o shm_bug32
shm_bug.c: In function ‘main’:
shm_bug.c:12:24: warning: initialization makes pointer from integer without a 
cast [-Wint-conversion]
       const void *at = 0x7f7df38ea000;
                        ^~~~~~~~~~~~~~
~/qemu$ i386-linux-user/qemu-i386 ./shm_bug32 
got err 0, ptr 0xffffffff
ari@ari-thinkpad:~/qemu$ gcc shm_bug.c -o shm_bug64
shm_bug.c: In function ‘main’:
shm_bug.c:12:24: warning: initialization makes pointer from integer without a 
cast [-Wint-conversion]
       const void *at = 0x7f7df38ea000;
                        ^~~~~~~~~~~~~~
~/qemu$ x86_64-linux-user/qemu-x86_64 ./shm_bug64 
got err 0, ptr 0x7f7df38ea000
ari@ari-thinkpad:~/qemu$ 

Additionally, running each executable directly on a 64-bit Ubuntu 18.04
system, we can see that the behavior of the 32-bit binary differs
between qemu-i386 and native, while that of the 64-bit binary does not:

~/qemu$ ./shm_bug32 
got err 0, ptr 0xf38ea000
~/qemu$ ./shm_bug64
got err 0, ptr 0x7f7df38ea000
~/qemu$ 


** Changed in: qemu
       Status: Expired => Confirmed

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https://bugs.launchpad.net/bugs/1462640

Title:
  shmat fails on 32-to-64 setup

Status in QEMU:
  Confirmed

Bug description:
  
  I am trying to run a guest mips32 program (user mode) on a x86_64 host. The 
program fails on a call to shmat() reproducibly. when digging into this 
problem, I could make a small guest POC that fails when compiled as i386 (-m32) 
running on a x86_64 host, but pass when compiled as 64bit. The problem has to 
do with mmap flags.

  From what I can understand, when running 32bits guests programs, qemu
  reserve the whole guest virtual space with an mmap call. That mmap
  call specifys MAP:PRIVATE flag. When shmat is called, it tries to make
  part of that region MAP_SHARED and that fails.

  As a possible fix, it looks like it is possible to first unmap the shm
  region before calling shmat.

  steps to reproduce: 
  1 - create a file shm.c with content below
  2 - compile with: gcc -m32 shm.c -o shm32
  3 - run on a x86_64 host: qemu-i386 ./shm32 
  4 - observe shmat fails, by returning ptr -1

  5- compile without -m32: : gcc shm.c -o shm64
  6 - observe it pass: qemu-x84_64 ./shm64


  #include <sys/ipc.h>
  #include <sys/shm.h>
  #include <sys/mman.h>
  #include <stdio.h>

  int main()
  {
      struct shmid_ds shm_desc;
      int err = 0;
      int id = shmget(IPC_PRIVATE, 688128, IPC_CREAT|IPC_EXCL|0666);
      err = shmctl(id, IPC_STAT, &shm_desc);
      const void *at = 0x7f7df38ea000;
      void* ptr = shmat(id, at, 0);
      printf( "got err %d, ptr %p\n", err, ptr );
  }

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