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Message-ID: <52E757A1-C021-41E6-8157-A2C9252B5FA2@koorogi.info>
Date: Fri, 09 Jun 2023 12:40:46 -0500
From: Bobby Bingham <koorogi@...rogi.info>
To: musl@...ts.openwall.com, Rich Felker <dalias@...c.org>,
Tali Auster <tali.auster@...il.com>
Subject: Re: execve with null envp
The Linux manpage explicitly says to not rely on this and that other systems are known to return EFAULT in this case.
https://www.man7.org/linux/man-pages/man2/execve.2.html#NOTES
On June 9, 2023 12:04:04 PM CDT, Rich Felker <dalias@...c.org> wrote:
>On Fri, Jun 09, 2023 at 09:05:15AM -0600, Tali Auster wrote:
>> Hello,
>>
>> Is there any specific behavior specified when NULL is passed for
>> envp in any of the exec* methods that accept it? I took a quick look
>> at the source tree, and process/execve.c has a comment suggesting
>> that the correct behavior may be to use environ for envp before
>> passing the NULL to the syscall unchanged. My intuition - and, in
>> practice, Linux with glibc seems to agree - is that a NULL envp
>> means an empty environment. I imagine the answer to this question
>> may just be a vague shrug, but does POSIX mandate anything specific
>> here? Or, perhaps a question with a more clear answer, can my
>> userspace code call execve(pathname, argv, NULL) and reliably expect
>> the child to have an empty environment?
>>
>> Hopefully this is on-topic for this list; regardless, thanks for any
>> help or direction y'all can provide!
>
>The only specification I'm aware of is:
>
> "The argument envp is an array of character pointers to
> null-terminated strings. These strings shall constitute the
> environment for the new process image. The envp array is
> terminated by a null pointer."
>
>So, if the caller fails to satisfy that part of the interface
>contract, the behavior is undefined.
>
>I'm not sure if there's any historical consistency on what happens
>here or if any implementations have the behavior the comment suggested
>might be "right", but short of research to establish that there's a
>reliable behavior and effort to standardize it, I think the clear
>solution here is "don't do that". It's easy to just pass (char*[]){0}
>instead of a null pointer.
>
>Rich
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