linux-kernel - Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

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Message-ID: <20211105184556.GN174703@worktop.programming.kicks-ass.net>
Date:   Fri, 5 Nov 2021 19:45:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        mark.rutland@....com, dvyukov@...gle.com, pbonzini@...hat.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 07/22] x86,extable: Extend extable functionality

On Fri, Nov 05, 2021 at 05:32:14PM +0000, Sean Christopherson wrote:

> > +#define EX_IMM_MASK	0xFFFF0000

> > +	imm  = FIELD_GET(EX_IMM_MASK,  e->type);
> 
> FIELD_GET casts the result based on the type of the mask, but doesn't explicitly
> sign extend the masked field, i.e. there's no intermediate cast to tell the compiler
> that the imm is a 16-bit value that should be sign extended.
> 
> Modifying FIELD_GET to sign extended is probably a bad idea as I'm guessing the
> vast, vast majority of use cases don't want that behavior.  I'm not sure how that
> would even work with masks that are e.g. 5 bits or so.

So the way I was reading it was that typeof(_mask) is 'int', e->type is
also 'int', we mask out the top bits, and since it's all 'int' we do an
arith shift right (ie. preserves sign).

Where did that reading go wrong?