linux-kernel - Re: Don't fill the kernel log with memfd

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Date:   Tue, 5 Sep 2023 14:10:04 +0200
From:   "Linux regression tracking #update (Thorsten Leemhuis)" 
        <regressions@...mhuis.info>
To:     Linux kernel regressions list <regressions@...ts.linux.dev>
Cc:     stable@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: Don't fill the kernel log with memfd_create messages

[TLDR: This mail in primarily relevant for Linux kernel regression
tracking. See link in footer if these mails annoy you.]

On 04.09.23 15:31, Vlastimil Babka wrote:
> On 8/30/23 17:52, Alex Xu (Hello71) wrote:
>> Hi all,
>>
>> Recently "memfd: improve userspace warnings for missing exec-related 
>> flags" was merged. On my system, this is a regression, not an 
>> improvement, because the entire 256k kernel log buffer (default on x86) 
>> is filled with these warnings and "__do_sys_memfd_create: 122 callbacks 
>> suppressed". I haven't investigated too closely, but the most likely 
>> cause is Wayland libraries.
>>
>> This is too serious of a consequence for using an old API, especially 
>> considering how recently the flags were added. The vast majority of 
>> software has not had time to add the flags: glibc does not define the 
>> macros until 2.38 which was released less than one month ago, man-pages 
>> does not document the flags, and according to Debian Code Search, only 
>> systemd, stress-ng, and strace actually pass either of these flags.
>>
>> Furthermore, since old kernels reject unknown flags, it's not just a 
>> matter of defining and passing the flag; every program needs to 
>> add logic to handle EINVAL and try again.
>>
>> Some other way needs to be found to encourage userspace to add the 
>> flags; otherwise, this message will be patched out because the kernel 
>> log becomes unusable after running unupdated programs, which will still 
>> exist even after upstreams are fixed. In particular, AppImages, 
>> flatpaks, snaps, and similar app bundles contain vendored Wayland 
>> libraries which can be difficult or impossible to update.
> 
> It's being reverted:
> https://lore.kernel.org/all/20230902230530.6B663C433C8@smtp.kernel.org/

in that case:

#regzbot fix: revert "memfd: improve userspace warnings for missing
exec-related flags".
#regzbot ignore-activity

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
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