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

[Andreas Schwab <schwab@linux-m68k.org>]

On Apr 13 2021, Adhemerval Zanella wrote:

> 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

I'd still use 6 * 1024 * 1024 here, small numbers are much easier to
grok.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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