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

[Michael Matz <matz@suse.de>]

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".

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.