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Date: Thu, 25 Apr 2019 10:32:28 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [patch] return value of ulimit(UL_GETFSIZE) in X32
 architecture

On Thu, Apr 25, 2019 at 01:44:24PM +0000, liucheng (G) wrote:
> Dear ALL,
> 
> Return value of ulimit(UL_GETFSIZE) in X32 architecture seems to be wrong.

Is this x32-specific? Unless I'm mistaken this is always the case when
long is 32-bit. POSIX notes in APPLICATION USAGE:

    "Since the ulimit() function uses type long rather than rlim_t,
    this function is not sufficient for file sizes on many current
    systems. Applications should use the getrlimit() or setrlimit()
    functions instead of the obsolescent ulimit() function."

http://pubs.opengroup.org/onlinepubs/9699919799/functions/ulimit.html

and marks the function obsolescent, without actually specifying what
it should return in this case. A bug should probably be opened for
clarification on that, but applications definitely should not be using
the ulimit function.

> Here is the implementation of ulimit function in MUSL 1.1.22:
> #include <sys/resource.h>
> #include <ulimit.h>
> #include <stdarg.h>
> 
> long ulimit(int cmd, ...)
> {
>                  struct rlimit rl;
>                  getrlimit(RLIMIT_FSIZE, &rl);
>                  if (cmd == UL_SETFSIZE) {
>                          long val;
>                          va_list ap;
>                          va_start(ap, cmd);
>                          val = va_arg(ap, long);
>                          va_end(ap);
>                          rl.rlim_cur = 512ULL * val;
>                                      if (setrlimit(RLIMIT_FSIZE, &rl)) return -1;
>                  }
>                  return rl.rlim_cur / 512;
> }
> 
> Make it simple in case of "cmd = UL_GETFSIZE", ulimit function becomes to be:
>          long ulimit(int cmd, ...)
>          {
>                    struct rlimit rl;
>                    getrlimit(RLIMIT_FSIZE, &rl);
>                    return rl.rlim_cur / 512;
> }
> 
> rl.rlim in ulimit function is the type of long long(8 Byte), however the return value in X32 architecture is the type of long(4 Byte).
> So in that case, rl.rlim_cur / 512 would be larger than 0x7fffffff and ulimit function returns -1.
> 
> 
> I also tried an experiment to improve my opinion as follows.
> 
> [benchmark]
> $ cat ulimit_test.c
> #include <stdio.h>
> #include <ulimit.h>
> 
> int main()
> {
>         printf("ret:%d\n", ulimit(UL_GETFSIZE));
> 
>         return 0;
> }
> 
> [testcase]
> Environment: Linux 4.4.171 #1 SMP Thu Apr 25 00:39:22 UTC 2019 armv7l GNU/Linux
> 
> $ /tmp # ulimit -f
> unlimited
> $ /tmp # ./ulimit_test
> ret:-1
> $ /tmp # cp musl/libc.so /usr/lib/                                 // copy a new MUSL libc.so with my [patch]
> $ /tmp # ./ulimit_test
> ret:2147483647                                                               // 0x7fffffff
> 
> [patch]
> ---
> diff --git a/src/legacy/ulimit.c b/src/legacy/ulimit.c
> index 1f59e8e..d1620e6 100644
> --- a/src/legacy/ulimit.c
> +++ b/src/legacy/ulimit.c
> @@ -1,6 +1,7 @@
> #include <sys/resource.h>
> #include <ulimit.h>
> #include <stdarg.h>
> +#include <limits.h>
>  long ulimit(int cmd, ...)
> {
> @@ -15,5 +16,5 @@ long ulimit(int cmd, ...)
>                 rl.rlim_cur = 512ULL * val;
>                 if (setrlimit(RLIMIT_FSIZE, &rl)) return -1;
>        }
> -        return rl.rlim_cur / 512;
> +       return rl.rlim_cur == RLIM_INFINITY ? LONG_MAX : rl.rlim_cur / 512;
> }
> ---

The value LONG_MAX isn't correct either, since you can obviously
create files larger than LONG_MAX. The "morally" correct behavior is
probably returning -1 with an implementation-defined (unless/until
POSIX standardizes it) of EOVERFLOW, the error used for similar
interfaces (e.g. ftell) when a file size does not fit in long.

Oddly, there's also no specification for what it should return when
the limit actually is infinite.

Rich

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