[PATCH] x86: Warn .insn instruction with length > 15 bytes

[H.J. Lu]

On Tue, Feb 6, 2024 at 7:48 AM Michael Matz <matz@suse.de> wrote:
>
> Hello,
>
> On Tue, 6 Feb 2024, H.J. Lu wrote:
>
> > > > > > It is an error on both Intel and AMD processors.   There is no
> > > > > > valid reason not to be an error at the moment.
> > > > >
> > > > > Jan gave you one.  I would prefer for the assembler to not be anally
> > > >
> > > > That is not a valid reason.
> > >
> > > Yes it is.
> > >
> > > > There is no such processor in the foreseeable future.
> > >
> > > Doesn't matter.
> > >
> >
> > When a warning is given, a decodable instruction should still
> > be generated.
>
> A software library can decode such instruction just fine.  A human as well.
>
> > Assembler shouldn't generate something which
> > can't be decoded by default.
>
> That's the important words: "by default".  Here's the documentation for
> as_bad and as_warn:
>
>    as_bad() is used to mark errors that result in what we
>    presume to be a useless object file.  Say, we ignored
>    something that might have been vital.
>    ...
>
>    as_warn() is used when we have an error from which we
>    have a plausible error recovery.  eg, masking the top
>    bits of a constant that is longer than will fit in the
>    destination.  In this case we will continue to assemble
>    the source, although we may have made a bad assumption,
>    and we will produce an object file and return normal exit
>    status (ie, no error).
>    ...
>
> It's obvious to me that just continuing to assemble the over-long
> instruction is a "plausible error recovery".  It's even more plausible
> than "masking the top bits of a constant".  Certainly an object file
> containing a byte sequence correspending to the overlong instruction is
> not "useless".

With as_bad, assembler will continue to assemble, just not generate
an object file.   We ran into this with APX.  Not everyone checks
assembler warnings closely.  It led to mysterious crashes.   I am
not against it if someone else implements an assembler option to
turn this error into a warning.

> I understand why you want to give an error by default, even though I
> disagree with even that (in my book only a warning is justified).  But
> ruling out that this can be demoted to a warning, possibly with an option,
> is not in line with my expectation of how the GNU assembler should work
> and has traditionally worked.
>
>
> Ciao,
> Michael.



-- 
H.J.