Re: obscure bug "extern void free (void *__ptr) __attribute__ ((__nothrow__ , __leaf__));"

[Eduardo A . Bustamante López]

On Fri, Jan 22, 2021 at 05:05:31AM +0100, Mathias Steiger wrote:
> 
> Here is the strace output.
> 
> https://filebin.net/9auqyreezma08z12/bug_bash.tar.gz?t=3bjx4xpd
> 
> It is very excessive due to the nature of Autoconf and I couldn't make a lot
> of sense of it.
> 
> 
> When I downgraded the package from bash-5.1.004-1 to bash-5.0.018-2 the bug
> disappeared.

This trace file is quite interesting (redacted for brevity):

$ cat ./strace_if-statement_removed/strace_second.1243868 
(...)
openat(AT_FDCWD, "./config.status", O_WRONLY|O_CREAT|O_APPEND, 0666) = 8
dup2(8, 1)                              = 1
close(8)                                = 0
openat(AT_FDCWD, "/dev/null", O_RDONLY) = 8
dup2(8, 0)                              = 0
close(8)                                = 0
execve("/bin/cat", ["cat"], [(...)]) = 0
(...)
fstat(1, {st_dev=makedev(0xfe, 0x3), st_ino=42548069, st_mode=S_IFREG|0775, 
st_nlink=1, st_uid=1006, st_gid=0, st_blksize=4096, st_blocks=32, 
st_size=16357, st_atime=1611282495 /* 2021-01-22T03:28:15.296486563+0100 */, 
st_atime_nsec=296486563, st_mtime=1611282560 /* 
2021-01-22T03:29:20.712747829+0100 */, st_mtime_nsec=712747829, 
st_ctime=1611282560 /* 2021-01-22T03:29:20.712747829+0100 */, 
st_ctime_nsec=712747829}) = 0
fstat(0, {st_dev=makedev(0, 0x5), st_ino=1728, st_mode=S_IFREG|0666, 
st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=8, st_size=73, 
st_atime=1611282495 /* 2021-01-22T03:28:15.243153560+0100 */, 
st_atime_nsec=243153560, st_mtime=1611282549 /* 
2021-01-22T03:29:09.822815317+0100 */, st_mtime_nsec=822815317, 
st_ctime=1611282549 /* 2021-01-22T03:29:09.822815317+0100 */, 
st_ctime_nsec=822815317}) = 0
(...)
read(0, "extern void free (void *__ptr) __attribute__ ((__nothrow__ , 
__leaf__));\n", 131072) = 73
write(1, "extern void free (void *__ptr) __attribute__ ((__nothrow__ , 
__leaf__));\n", 73) = 73
read(0, "", 131072)                     = 0
(...)


This is in essence:

  cat </dev/null >./config.status

These are worth highlighting:

- Notice that the value returned by `fstat(0, ...)' indicates that /dev/null in 
your system is a *regular* file (it should be `st_mode=S_IFCHR|0666', but 
instead it is `st_mode=S_IFCHR|0666'). It also indicates that its size is 73 
bytes (`st_size=73').
- Notice that `cat' reads from file descriptor 0 (i.e. /dev/null), and the 
return value is a string of 73 bytes in length.
- `cat' then writes that string out to `./config.status'

73 happens to be the length of the string that has been causing issues for you:

  $ echo "extern void free (void *__ptr) __attribute__ ((__nothrow__ , 
__leaf__));" | wc -c
  73

Can you run:

  stat /dev/null

And share with us what you get?

This is how it should look like:

  $ stat /dev/null
    File: /dev/null
    Size: 0             Blocks: 0          IO Block: 4096   character special 
file
  Device: 5h/5d Inode: 4           Links: 1     Device type: 1,3
  Access: (0666/crw-rw-rw-)  Uid: (    0/    root)   Gid: (    0/    root)
  Access: 2021-01-09 15:28:48.589801534 -0800
  Modify: 2021-01-09 15:28:48.589801534 -0800
  Change: 2021-01-09 15:28:48.589801534 -0800
   Birth: -