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Re: [PATCH] Fix null environ crash in getenv() provided by lib/sh/getenv

From: Eric Blake
Subject: Re: [PATCH] Fix null environ crash in getenv() provided by lib/sh/getenv.c
Date: Tue, 24 Jul 2018 09:14:56 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

On 07/09/2018 09:16 AM, Chet Ramey wrote:
On 7/8/18 7:21 AM, Keeley Hoek wrote:
In running bash on an embedded target, bash crashes for the silly
reason that environ is NULL.

I haven't been able to tell whether this is actually permitted by the
standard (as if?), but in bash I think this behavior is inconsistent
anyway because:

It's probably permitted by the standard simply because the standard doesn't
specify the value of environ. As a practical matter, on Unix systems, even
when execve() is supplied a NULL third argument, environ is never null.
It's not unreasonable to check for it, though.

It's unspecified by the standard, so portable programs should not assume it works. The only portable thing for emptying the environment is to assign environ to a 1-element array containing a NULL pointer - but even that is not portable as a completely empty environment wipes out PATH and possibly other important variables (such as POSIXLY_CORRECT) that may subtly break expectations.

That said, Cygwin was just recently patched to allow environ=NULL as shorthand for emptying the environment:

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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