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Re: Violations of mandatory MISRA C:2012 Rule 19.1 in X86_64 build
- To: Roberto Bagnara <roberto.bagnara@xxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 12 Jul 2023 14:49:51 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 12 Jul 2023 12:50:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 11.07.2023 18:40, Roberto Bagnara wrote:
> Mandatory Rule 19.1 (An object shall not be assigned or copied to an
> overlapping object) is directly targeted at two undefined behaviors,
> one of which is the subject of 6.5.16.1p3, namely:
>
> If the value being stored in an object is read from another object
> that overlaps in any way the storage of the first object, then the
> overlap shall be exact and the two objects shall have qualified or
> unqualified versions of a compatible type; otherwise, the behavior
> is undefined.
>
> You can see a number of definite violations in the X86_64 build
> at this link:
>
>
> https://saas.eclairit.com:3787/fs/var/local/eclair/XEN.ecdf/ECLAIR_normal/origin/staging/X86_64-Set1/149/PROJECT.ecd;/by_service/MC3R1.R19.1.html
Hmm, gives me "Unauthorized".
> As the rule is mandatory, it cannot be deviated.
Which is odd, and that's in turn because the C spec is overly strict
imo. Considering the one example we looked at yesterday, I have a hard
time seeing what UB there would be on little-endian hardware. Both
fields (rip and eip) don't overlap exactly, but their low halves (which
is all we care about here) do. With this I cannot come up with a way in
which the compiler could screw us up, even if it wanted to.
Jan
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