Re: [PATCH] linux: sysconf: limit _SC_MAX_ARG to 6 MiB [BZ #25305]

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 13/04/2021 09:13, Andreas Schwab wrote:
> On Apr 12 2021, Michal Nazarewicz wrote:
> 
>>> On 07/04/2021 12:10, Michal Nazarewicz wrote:
>>>> Since Linux 4.13, kernel limits the maximum command line arguments
>>>> length to 6 MiB.¹  Normally the limit is still quarter of the maximum
>>>> stack size but if that limit exceeds 6 MiB it’s clamped down.
>>
>> On Mon, Apr 12 2021, Adhemerval Zanella wrote:
>>> Patch look good to me, thanks.
>>>
>>> Reviewed=by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
>>>
>>>> ---
>>>>  sysdeps/unix/sysv/linux/sysconf.c | 5 ++++-
>>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/sysdeps/unix/sysv/linux/sysconf.c b/sysdeps/unix/sysv/linux/sysconf.c
>>>> index 366fcef01e..bd711795c7 100644
>>>> --- a/sysdeps/unix/sysv/linux/sysconf.c
>>>> +++ b/sysdeps/unix/sysv/linux/sysconf.c
>>>> @@ -55,7 +55,10 @@ __sysconf (int name)
>>>>          struct rlimit rlimit;
>>>>          /* Use getrlimit to get the stack limit.  */
>>>>          if (__getrlimit (RLIMIT_STACK, &rlimit) == 0)
>>>> -	  return MAX (legacy_ARG_MAX, rlimit.rlim_cur / 4);
>>>> +	  {
>>>> +	    const long int limit = MAX (legacy_ARG_MAX, rlimit.rlim_cur / 4);
>>>> +	    return MIN (limit, 6 << 10 << 10);
>>>
>>> I think it a bit easier to read with the value expanded (6291456).
>>
>> I’d rather go with ‘6 * 1024 * 1024’ if shifts aren’t readable.
> 
> In addition, I'd give it a symbolic name with a comment.

What about:

diff --git a/sysdeps/unix/sysv/linux/sysconf.c b/sysdeps/unix/sysv/linux/sysconf.c
index 366fcef01e..aceedfa87c 100644
--- a/sysdeps/unix/sysv/linux/sysconf.c
+++ b/sysdeps/unix/sysv/linux/sysconf.c
@@ -33,6 +33,9 @@
    actual value varies based on the stack size.  */
 #define legacy_ARG_MAX 131072
 
+/* Newer kernels (4.13) limit the maximum command line arguments lengths to
+   6MiB.  */
+#define maximum_ARG_MAX 6291456
 
 static long int posix_sysconf (int name);
 
@@ -55,7 +58,10 @@ __sysconf (int name)
         struct rlimit rlimit;
         /* Use getrlimit to get the stack limit.  */
         if (__getrlimit (RLIMIT_STACK, &rlimit) == 0)
-         return MAX (legacy_ARG_MAX, rlimit.rlim_cur / 4);
+         {
+           const long int limit = MAX (legacy_ARG_MAX, rlimit.rlim_cur / 4);
+           return MIN (limit, maximum_ARG_MAX);
+         }
 
         return legacy_ARG_MAX;
       }