linux-kernel - Re: [PATCH] devtmpfs: don't use vfs_getattr_nosec to query i

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Message-ID: <D865215C-0373-464C-BB7D-235ECAF16E49@kernel.org>
Date: Fri, 25 Apr 2025 08:40:23 -0700
From: Kees Cook <kees@...nel.org>
To: Christoph Hellwig <hch@....de>, Christian Brauner <brauner@...nel.org>
CC: Heiko Carstens <hca@...ux.ibm.com>, gregkh@...uxfoundation.org,
 rafael@...nel.org, dakr@...nel.org, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org,
 Shin'ichiro Kawasaki <shinichiro.kawasaki@....com>, Xiao Ni <xni@...hat.com>
Subject: Re: [PATCH] devtmpfs: don't use vfs_getattr_nosec to query i_mode



On April 25, 2025 6:32:59 AM PDT, Christoph Hellwig <hch@....de> wrote:
>On Fri, Apr 25, 2025 at 12:12:36PM +0200, Christian Brauner wrote:
>> > That is: if dev_mynode(dev, inode) is not true some random value will be returned.
>> 
>> Don't bother resending, Christoph.
>> I've already fixed this with int err = 0 in the tree.
>
>Thanks!  Let me use this as a platform to rant about our option
>defaults and/or gcc error handling.  It seems like ever since we started
>zeroing on-stack variables by default gcc stopped warnings about using
>uninitialized on-stack variables, leading to tons of these case where
>we don't catch uninitialized variables.  Now in this and in many cases
>the code works fine because it assumed zero initialization, but there are
>also cases where it didn't, leading to new bugs.

This isn't the case: the feature was explicitly designed in both GCC and Clang to not disrupt -Wuninitialized. But -Wuninitialized has been so flakey for so long that it is almost useless (there was even -Wmaybe-uninitialized added to try to cover some of the missed diagnostics). And it's one of the many reasons stack variable zeroing is so important, since so much goes undiagnosed. :(

>Can we fix this somehow?

Fixing -Wuninitialized would be lovely, but it seems no one has been able to for years now. 😭

-- 
Kees Cook