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

[Oleg Andriyanov]

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

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

Status in QEMU:
  New

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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