linux-kernel - Re: [PATCH v4] execve.2: execve also returns E2BIG if a string is too long

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Message-ID: <ZScCP2RqArDeixLz@debian>
Date:   Wed, 11 Oct 2023 22:14:55 +0200
From:   Alejandro Colomar <alx@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-man@...r.kernel.org, kernel-team@...a.com,
        linux-kernel@...r.kernel.org,
        Matthew House <mattlloydhouse@...il.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH v4] execve.2: execve also returns E2BIG if a string is
 too long

Hi Rik,

On Wed, Oct 11, 2023 at 12:53:30PM -0400, Rik van Riel wrote:
> Sigh, once again I did a git commit --amend without the latest file change being
> included. The change below should be good. Working with both git and hg gets me sometimes :/
> ---8<---
> 
> The execve syscall returns -E2BIG in 3 cases:
> - The total length of the command line arguments and environment is too large.
> - An argument or environment string is longer than MAX_ARG_STRLEN.

I'm still concerned by the wording of this sentence (and the next one).
When I (and any user-space programmer, I think) read "string is longer
than FOO", I read `strlen(string) > FOO`.  Since it's rather
`strlen(string) >= FOO`, I think you should clarify.  How about this?

-  An argument or environment string is longer (including the null byte)
   than MAX_ARG_STRLEN.

Thanks,
Alex

> - The full path to the executable exceeds MAX_ARG_STRLEN.
> 
> Spell out all 3 cases in the -E2BIG section.
> 
> Discovered by moving a too large commandline parameter to an environment
> variable, and finding that things still did not work. Examined the code
> in fs/exec.c to get the details.
> 
> This simple shell script starts failing at 2^17 on a system with 4kB
> page size:
> ./exec2big.sh: line 10: /bin/true: Argument list too long
> fork failed at loop 17
> 
> STRING="a"
> 
> for loop in `seq 20`; do
> 	STRING="$STRING$STRING"
> 	export STRING
> 	if /bin/true ; then
> 		: # still under the limit
> 	else
> 		echo "fork failed at loop $loop"
> 	fi
> done

Thank you very much!

> 
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Suggested-by: Matthew House <mattlloydhouse@...il.com>
> ---
>  man2/execve.2 | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/man2/execve.2 b/man2/execve.2
> index 0d9582492ad1..b689101771e5 100644
> --- a/man2/execve.2
> +++ b/man2/execve.2
> @@ -449,7 +449,12 @@ The total number of bytes in the environment
>  .RI ( envp )
>  and argument list
>  .RI ( argv )
> -is too large.
> +is too large,
> +an argument or environment string is too long,
> +or the full
> +.I pathname
> +of the executable is too long.
> +The terminating NUL is counted as part of the string length.
>  .TP
>  .B EACCES
>  Search permission is denied on a component of the path prefix of
> -- 
> 2.41.0
> 
> 

-- 
<https://www.alejandro-colomar.es/>

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