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

From: Laurent Vivier
Subject: [Qemu-devel] [Bug 1462640] Re: shmat fails on 32-to-64 setup
Date: Fri, 05 Jul 2019 16:13:09 -0000

Mapping an arbitrary address in the memory takes the risk to have this address 
already in use by the loader or a library.
Where does this address come from?
Why the qemu-mips program doesn't call shmat() with NULL?

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  shmat fails on 32-to-64 setup

Status in QEMU:

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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