linux-kernel - Re: [PATCH v5 14/14] selftests/nolibc: add mmap and munmap test cases

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Date:   Sun, 2 Jul 2023 21:33:06 +0200
From:   Willy Tarreau <w@....eu>
To:     Zhangjin Wu <falcon@...ylab.org>
Cc:     thomas@...ch.de, arnd@...db.de, david.laight@...lab.com,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Thomas Weißschuh <linux@...ssschuh.net>
Subject: Re: [PATCH v5 14/14] selftests/nolibc: add mmap and munmap test cases

Hi Zhangjin,

On Wed, Jun 28, 2023 at 09:51:57PM +0800, Zhangjin Wu wrote:
> Three mmap/munmap related test cases are added:
> 
> - mmap_bad: the length argument must be greater than 0, otherwise, fail
>   with -EINVAL.
> 
> - munmap_bad: invalid (void *)-1 address fail with -EINVAL.
> 
> - mmap_munmap_good: mmap() a file with good offset and then munmap().
> 
> Note, it is not easy to find a unique file for mmap() in different
> scenes, so, a file list is used to search the right one:
> 
> - /proc/1/exe, for 'run' and 'run-user' target
>   'run-user' can not find '/proc/self/exe'
> 
> - /proc/self/exe, for 'libc-test' target
>   normal program 'libc-test' has no permission to access '/proc/1/exe'

Strictly speaking, if your executable is not readable (e.g. chmod 111
due to a restrictive umask) it will also fail that one.

> - the others, for kernel without procfs
>   let it pass even with 'worst case' kernel configs

You should include /dev/zero, which is commonly used to allocate anonymous
memory and is more likely present and readable than any of the other files.
And another file of choice is obviously argv[0] ;-)  In this case you don't
need any of the other extra ones. Thus I could suggest that you try in this
order:

    /dev/zero, /proc/self/exe, /proc/1/exe, argv[0]

and be done with it. That doesn't prevent one from extending the list if
really needed later, but I doubt it would be needed. Also, it's already
arranged in a read-write, then read-only fallbacks mode, so if we later
need to add more complex tests involving writes, the writable /dev/zero
will have precedence.

Willy