[Bug 1794086] Re: readlink(2) returns incorrect size for /proc/self/exe

[Thomas Huth]

The QEMU project is currently considering to move 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 older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch the state 
back to "New" within the next 60 days, otherwise this report will be marked as 
"Expired". Or mark it as "Fix Released" if the problem has been solved with a 
newer version of QEMU already. Thank you and sorry for the inconvenience.

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

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https://bugs.launchpad.net/bugs/1794086

Title:
  readlink(2) returns incorrect size for /proc/self/exe

Status in QEMU:
  Incomplete

Bug description:
  readlink(2) seems to ignore the size of supplied buffer for the
  resolved name and always returns the actual size of the resolved name
  instead.

  Steps to reproduce:

  ```bash
  echo '#include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>

  int main(int argc, const char** argv)
  {
      if(argc < 2) exit(1);
      char buf[1];
      printf("%d\n", readlink(argv[1], buf, sizeof(buf)));
  }' >test.c

  # I used GCC mipsel cross-compiler to reproduce this bug
  mipsel-linux-gnu-gcc-5.5 test.c -o a.out

  echo "PWD: `pwd`"
  qemu-mipsel ./a.out /proc/self/exe
  ```

  Expected output (observed when running a.out natively on Linux 4.17 amd64):
  ```
  PWD: /tmp/test
  1
  ```

  Output observed when running with qemu-mipsel 2.1.2:
  ```
  PWD: /tmp/test
  15
  ```

  According to POSIX description of readlink [1], the function shall
  return the number of bytes written to the supplied buffer, which
  obviously cannot exceed size of the buffer.

  Note that the bug is only reproduced with links within /proc
  filesystem; links to the regular files within /home are resolved
  normally.

  The bug is present in qemu-mipsel 2.1.2:

  # qemu-mipsel -version
  qemu-mipsel version 2.1.2 (Debian 1:2.1+dfsg-12+deb8u6), Copyright (c) 
2003-2008 Fabrice Bellard

  [1]:
  http://pubs.opengroup.org/onlinepubs/009695399/functions/readlink.html

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