RFC: Rename visible libmagic symbols

[orlitzky]

Trying my hand at a very old problem (PHP bug 66095, Gentoo bug 471682, FreeBSD bug 195562)

The bundled libmagic for the fileinfo extension has some visible symbols:

$ objdump -TC /usr/lib/php8.5/apache2/libphp8.so | grep '\smagic_'
00000000002b3f34 g    DF .text	0000000000000034  Base        magic_compile
00000000002b3dde g    DF .text	0000000000000024  Base        magic_open
00000000002b3fd0 g    DF .text	0000000000000034  Base        magic_descriptor
00000000002b3f00 g    DF .text	0000000000000034  Base        magic_load
00000000002b43fa g    DF .text	0000000000000032  Base        magic_setflags
00000000002b3f9c g    DF .text	0000000000000034  Base        magic_list
00000000002b4442 g    DF .text	0000000000000206  Base        magic_setparam
00000000002b43d4 g    DF .text	0000000000000026  Base        magic_getflags
00000000002b439e g    DF .text	0000000000000036  Base        magic_errno
00000000002b4648 g    DF .text	00000000000001f4  Base        magic_getparam
00000000002b42f4 g    DF .text	000000000000006e  Base        magic_buffer
00000000002b4362 g    DF .text	000000000000003c  Base        magic_error
00000000002b3f68 g    DF .text	0000000000000034  Base        magic_check
00000000002b442c g    DF .text	0000000000000016  Base        magic_version
00000000002b3eda g    DF .text	0000000000000026  Base        magic_close
00000000002b4038 g    DF .text	0000000000000034  Base        magic_stream
00000000002b4004 g    DF .text	0000000000000034  Base        magic_file

This works fine until you load that shared library into a process that links against the system libmagic, providing (in many cases, different versions of) the same symbols. I hit this myself several years ago when using another apache module that linked against libmagic. I can't cite any bug reports for the embed SAPI, but there is every reason to suspect that it suffers from the same issue.

A fix was proposed on PHP bug 66095, but given how few problematic symbols there are, I think that it's easier to just rename them. I used find/sed and then checked the diff -- it was actually quite short. We can see the new names (the embed SAPI, in this case):

$ objdump -TC ./libs/libphp.so | grep php_magic
0000000000183aee g    DF .text	0000000000000036 php_magic_errno
0000000000183a44 g    DF .text	000000000000006e php_magic_buffer
0000000000183ab2 g    DF .text	000000000000003c php_magic_error
00000000001836b8 g    DF .text	0000000000000034 php_magic_check
0000000000183b7c g    DF .text	0000000000000016 php_magic_version
000000000018362a g    DF .text	0000000000000026 php_magic_close
0000000000183754 g    DF .text	0000000000000034 php_magic_file
0000000000183684 g    DF .text	0000000000000034 php_magic_compile
0000000000183788 g    DF .text	0000000000000034 php_magic_stream
0000000000183b4a g    DF .text	0000000000000032 php_magic_setflags
0000000000183720 g    DF .text	0000000000000034 php_magic_descriptor
0000000000183b92 g    DF .text	0000000000000206 php_magic_setparam
0000000000183b24 g    DF .text	0000000000000026 php_magic_getflags
000000000018352e g    DF .text	0000000000000024 php_magic_open
0000000000183d98 g    DF .text	00000000000001f4 php_magic_getparam
0000000000183650 g    DF .text	0000000000000034 php_magic_load
00000000001836ec g    DF .text	0000000000000034 php_magic_list

The strcasestr() commit is unrelated to an extent. I noticed (grep) that the only use of strcasestr() in libmagic is patched out by PHP, so there is no need for it on any platform. The -DHAVE_STRCASESTR=1 simply ensures that libmagic does not try to prototype it.

It remains to construct a failing example, and check that the PR actually fixes it.

[orlitzky]

Linking of the embed SAPI looks borked on master, but if I hack these patches onto the PHP-8.5 branch, pointlessly linking the embed SAPI into a small libmagic example induces the problem:

/* main.c */
#include <magic.h>
#include <stdio.h>

int main(int argc, char **argv)
{
  char *actual_file = "example.pdf";
  const char *magic_full;
  magic_t magic_cookie;

  magic_cookie = magic_open(MAGIC_MIME);
  if (magic_cookie == NULL) {
    printf("unable to initialize magic library\n");
    return 1;
  }

  printf("Loading default magic database...\n");
  if (magic_load(magic_cookie, "/usr/share/misc/magic.mgc") != 0) {
    printf("cannot load magic database - %s\n", magic_error(magic_cookie));
    magic_close(magic_cookie);
    return 1;
  }

  magic_full = magic_file(magic_cookie, actual_file);
  printf("libmagic answer: %s\n", magic_full);
  magic_close(magic_cookie);
  return 0;
}

Before the PR (my system copy of php-8.5):

$ gcc $(/usr/lib/php8.5/bin/php-config --includes) main.c -L/usr/lib/php8.5/lib -lphp -lmagic
$ LD_LIBRARY_PATH=/usr/lib/php8.5/lib ./a.out 
Segmentation fault         LD_LIBRARY_PATH=/usr/lib/php8.5/lib ./a.out

And the one I just built and installed to ~/tmp/php:

$ gcc $(/home/mjo/tmp/php/bin/php-config --includes) main.c -L/home/mjo/tmp/php/lib -lphp -lmagic
$ LD_LIBRARY_PATH=/home/mjo/tmp/php/lib ./a.out 
Loading default magic database...
libmagic answer: application/pdf; charset=binary

[orlitzky]

(example.pdf is a random PDF on my machine)