linux-kernel - Re: [PATCH v2] fs: always return zero on success from replace

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Message-ID: <20250805153457.GB222315@ZenIV>
Date: Tue, 5 Aug 2025 16:34:57 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Christian Brauner <brauner@...nel.org>
Cc: Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
	Jan Kara <jack@...e.cz>, Sargun Dhillon <sargun@...gun.me>,
	Kees Cook <kees@...nel.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] fs: always return zero on success from replace_fd()

On Tue, Aug 05, 2025 at 01:55:59PM +0200, Christian Brauner wrote:

> The calling conventions of do_dup2() are terrible. The only reason it
> drops file_lock itself instead of leaving it to the two callers that
> have to acquire it anyway is because it wants to call filp_close() if
> there's already a file on that fd.

Alternative calling conventions end up being nastier - I've tried.

> And really the side-effect of dropping a lock implicitly is nasty
> especially when the function doesn't even indicate that it does that in
> it's name.
> 
> And guards are great.

They do no allow to express things like "foo() consumes lock X".
>From time to time, we *do* need that, and when that happens guards
become a menace.

Another case is
	lock
	if (lock-dependent condition)
		some work
		unlock
		work that can't be under that lock
	else
		some other work
		unlock
		more work that can't be under that lock

Fairly common, especially when that's a spinlock and "can't be under that
lock" includes blocking operations.  Can't be expressed with guards, not
without a massage that often ends up with bloody awful results.