linux-kernel - RE: [PATCH 1/1] fs/qnx6: Fix building with GCC 15

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Message-ID: <665bcd89cf5f4679a38e9a84fa0ba42a@AcuMS.aculab.com>
Date: Sun, 6 Oct 2024 19:38:07 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Al Viro' <viro@...iv.linux.org.uk>, Brahmajit Das
	<brahmajit.xyz@...il.com>
CC: "brauner@...nel.org" <brauner@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/1] fs/qnx6: Fix building with GCC 15

...
> would explain what was really going on - the point is not to make gcc STFU, it's
> to make the code more straightforward.  The warning is basically "it smells
> somewhat fishy around >here<, might be worth taking a look".  And yes, it turned
> out to be fishy; minimal "make it STFU" would be to strip those NULs from
> the initializers (i.e. just go for static char match_root[2][3] = {".", ".."}; -
> an array initializer is zero-padded if it's shorter than the array), but that
> wasn't the only, er, oddity in that code.

Indeed - looks like it is checking that the first two directory entries
are "." and ".." in about the most complex way possible.

I have vague recollections on some code that ignored the first two entries
because they 'must be "." and ".."' - and then failed because some filesystem
(and I can't even remember the O/S) didn't meet its expectations!

A simple:
	if (strcmp(dir_entry[0].de_fname, ".") || strcmp(dir_entry[1].de_fname, ".."))
		error = 1;
would suffice.
The compiler ought to completely inline them.
On x86 to:
	error |= *(u16 *)dir_entry[0].de_fname ^ '.';
	error |= (*(u32 *)dir_entry[1].de_fname & 0xffffff) ^ ('.' * 0x101);
but I bet it doesn't!
(and it isn't quite the same)

	David.

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