linux-kernel - Re: [BUG][PATCH] arm64: bti: fix BTI to handle local indirect branches

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Date:   Tue, 6 Oct 2020 10:38:17 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [BUG][PATCH] arm64: bti: fix BTI to handle local indirect
 branches

On Mon, Oct 05, 2020 at 02:24:47PM -0500, Jeremy Linton wrote:
> Hi,
> 
> On 10/5/20 1:54 PM, Ard Biesheuvel wrote:
> >On Mon, 5 Oct 2020 at 20:18, Jeremy Linton <jeremy.linton@....com> wrote:
> >>
> >>The AES code uses a 'br x7' as part of a function called by
> >>a macro, that ends up needing a BTI_J as a target.
> >
> >Could we instead just drop the tail call, i.e, replace it with a ret
> >and do a 'bl' after it returns? The indirect call does not really
> >serve a purpose here anyway
> 
> Yes, that is an option, it adds an extra ret. Which probably doesn't mean
> much in most cases. I assumed this code was optimized this way because it
> mattered somewhere.

Since this really does seem to be a tail-call and since x16 and x17
appear to be otherwise unused here, can we not just use x16 or x17
instead of x7?

This relies on there being no other calls to veneered functions in the
mix, but this code is all in a single section so that shouldn't be a
concern.

Due to the magic status of x16 and x17 in br instructions, the resulting
jump should be compatible with BTI c.  I think this matches how the
compiler should typically compile tail-calls.

Cheers
---Dave