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From: Ian Jackson <iwj@xenproject.org>
To: Jane Malalane <jane.malalane@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	"Ian  Jackson" <iwj@xenproject.org>,
	Jan Beulich <jbeulich@suse.com>, Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH] xen/lib: Fix strcmp() and strncmp()
Date: Wed, 28 Jul 2021 11:42:57 +0100	[thread overview]
Message-ID: <24833.13489.362056.292831@mariner.uk.xensource.com> (raw)
In-Reply-To: <20210727184715.17699-1-jane.malalane@citrix.com>

Jane Malalane writes ("[PATCH] xen/lib: Fix strcmp() and strncmp()"):
> The C standard requires that each character be compared as unsigned
> char. Xen's current behaviour compares as signed char, which changes
> the answer when chars with a value greater than 0x7f are used.
> 
> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Signed-off-by: Jane Malalane <jane.malalane@citrix.com>

Thanks for this.

What are the practical effects of this bug ?  AFAICT in the hypervisor
code all the call sites simply test for zero/nonzero.

Of course we should fix this because

> -		if ((__res = *cs - *ct++) != 0 || !*cs++)

this substraction is UB if it overflows.  So in theory the compiler
could miscompile it - although in practice I can't see how the
assumption that this doesn't overflow would "help" the compiler.

Ian.


  reply	other threads:[~2021-07-28 10:43 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-27 18:47 [PATCH] xen/lib: Fix strcmp() and strncmp() Jane Malalane
2021-07-28 10:42 ` Ian Jackson [this message]
2021-07-30  6:52   ` Jane Malalane
2021-07-30  9:50     ` Ian Jackson
2021-07-30 10:12 ` Andrew Cooper
2021-07-30 12:29   ` Ian Jackson

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