linux-kernel - Re: [PATCH] fs: Use a cleanup attribute in copy

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Message-ID: <87a524jwg5.fsf@>
Date: Mon, 06 Oct 2025 09:55:38 +0200
From: Miquel Sabaté Solà <mssola@...ola.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org,  brauner@...nel.org,
  linux-kernel@...r.kernel.org,  jack@...e.cz
Subject: Re: [PATCH] fs: Use a cleanup attribute in copy_fdtable()

Al Viro @ 2025-10-05 22:30 +01:

> On Sun, Oct 05, 2025 at 07:41:47PM +0200, Miquel Sabaté Solà wrote:
>> Al Viro @ 2025-10-05 10:01 +01:
>>
>> > On Sun, Oct 05, 2025 at 07:37:50AM +0200, Miquel Sabaté Solà wrote:
>> >> Al Viro @ 2025-10-04 22:19 +01:
>> >>
>> >> > On Sat, Oct 04, 2025 at 11:03:40PM +0200, Miquel Sabaté Solà wrote:
>> >> >> This is a small cleanup in which by using the __free(kfree) cleanup
>> >> >> attribute we can avoid three labels to go to, and the code turns to be
>> >> >> more concise and easier to follow.
>> >> >
>> >> > Have you tried to build and boot that?
>> >>
>> >> Yes, and it worked on my machine...
>> >
>> > Unfortunately, it ends up calling that kfree() on success as well as on failure.
>> > Idiomatic way to avoid that would be
>> > 	return no_free_ptr(fdt);
>> > but you've left bare
>> > 	return fdt;
>> > in there, ending up with returning dangling pointers to the caller.  So as
>> > soon as you get more than BITS_PER_LONG descriptors used by a process,
>> > you'll get trouble.  In particular, bash(1) running as an interactive shell
>> > would hit that - it has descriptor 255 opened...
>>
>> Ugh, this is just silly from my end...
>>
>> You are absolutely right. I don't know what the hell I was doing while
>> testing that prevented me from realizing this before, but as you say
>> it's quite obvious and I was just blind or something.
>>
>> Sorry for the noise and thanks for your patience...
>
> FWIW, the real low-level destructor (__free_fdtable()) *does* cope with ->fd
> or ->open_fds left NULL, so theoretically we could replace kmalloc with
> kzalloc in alloc_fdtable(), add use that thing via DEFINE_FREE()/__free(...);
> I'm not sure if it's a good idea, though - at the very least, that property
> of destructor would have to be spelled out with explanations, both in
> __free_fdtable() and in alloc_fdtable().
>
> Matter of taste, but IMO it's not worth bothering with - figuring out why
> the damn thing is correct would take at least as much time and attention
> from readers as the current variant does.

Agreed.

>
> BTW, there's a chance to kill struct fdtable off - a project that got stalled
> about a year ago (see https://lore.kernel.org/all/20240806010217.GL5334@ZenIV/
> and subthread from there on for details) that just might end up eliminating
> that double indirect.  I'm not saying that it's a reason not to do cleanups in
> what exists right now, just a tangentially related thing that might be interesting
> to resurrect...

That looks interesting indeed, I'll take a look at this topic whenever I
have some spare time.

Thanks!
Miquel

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