linux-kernel - RE: [PATCH] KEYS: trusted: fix -Wvarags warning

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Message-ID: <0180360cfcb5458d8ff099744e8884d4@AcuMS.aculab.com>
Date:   Mon, 15 Oct 2018 09:26:02 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'ndesaulniers@...gle.com'" <ndesaulniers@...gle.com>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>
CC:     "natechancellor@...il.com" <natechancellor@...il.com>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] KEYS: trusted: fix -Wvarags warning

From: ndesaulniers@...gle.com
> Sent: 11 October 2018 21:31
...
> by swapping h2 and h3.
> 
> security/keys/trusted.c:146:17: warning: passing an object that
> undergoes default
>       argument promotion to 'va_start' has undefined behavior [-Wvarargs]
>         va_start(argp, h3);
>                        ^
> security/keys/trusted.c:126:37: note: parameter of type 'unsigned
> char' is declared here
> unsigned char *h2, unsigned char h3, ...)
>                                ^
> Specifically, it seems that both the C90 (4.8.1.1) and C11 (7.16.1.4)
> standards explicitly call this out as undefined behavior:

I guess that problems arise when all the arguments are stacked
and va_start/va_arg use naive pointer manipulation.
In that case &h3 might be 4n+3 aligned so va_arg() will access
misaligned stack locations.

I doubt any modern compilers (where va_start and va_arg are builtins)
will get this 'wrong' even when all arguments are stacked.

Seems clang is being over cautious.

	David

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