qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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


From: Ari Sundholm
Subject: [Qemu-devel] [Bug 1462640] Re: shmat fails on 32-to-64 setup
Date: Fri, 05 Jul 2019 15:37:30 -0000

For comparison, the strace output when running natively:

shmget(IPC_PRIVATE, 688128, IPC_CREAT|IPC_EXCL|0666) = 72843341
shmctl(72843341, IPC_64|IPC_STAT, {shm_perm={uid=1000, gid=1000, mode=0666, 
key=0, cuid=1000, cgid=1000}, shm_segsz=688128, shm_cpid=10883, shm_lpid=0, 
shm_nattch=0, shm_atime=0, shm_dtime=0, shm_ctime=1562340846}) = 0
shmat(72843341, 0xf38ea000, 0)          = 0xf38ea000
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 12), ...}) = 0
brk(NULL)                               = 0x58069000
brk(0x5808a000)                         = 0x5808a000
brk(0x5808b000)                         = 0x5808b000
write(1, "got err 0, ptr 0xf38ea000\n", 26got err 0, ptr 0xf38ea000
) = 26
exit_group(0)                           = ?
+++ exited with 0 +++
~/qemu$

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
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 );
  }

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1462640/+subscriptions



reply via email to

[Prev in Thread] Current Thread [Next in Thread]