Re: [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter

From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Dave Martin <Dave.Martin@arm.com>, Will Deacon <will@kernel.org>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Jeremy Linton <jeremy.linton@arm.com>,
	"H . J . Lu" <hjl.tools@gmail.com>,
	linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	libc-alpha@sourceware.org
Subject: Re: [PATCH v1 2/2] arm64: Enable BTI for main executable as well as the interpreter
Date: Thu, 3 Jun 2021 19:04:31 +0100	[thread overview]
Message-ID: <20210603180429.GI20338@arm.com> (raw)
In-Reply-To: <20210603165134.GF4257@sirena.org.uk>

On Thu, Jun 03, 2021 at 05:51:34PM +0100, Mark Brown wrote:
> On Thu, Jun 03, 2021 at 04:40:35PM +0100, Dave Martin wrote:
> > Do we know how libcs will detect that they don't need to do the
> > mprotect() calls?  Do we need a detection mechanism at all?
> > 
> > Ignoring certain errors from mprotect() when ld.so is trying to set
> > PROT_BTI on the main executable's code pages is probably a reasonable,
> > backwards-compatible compromise here, but it seems a bit wasteful.
> 
> I think the theory was that they would just do the mprotect() calls and
> ignore any errors as they currently do, or declare that they depend on a
> new enough kernel version I guess (not an option for glibc but might be
> for others which didn't do BTI yet).

I think we discussed the possibility of an AT_FLAGS bit. Until recently,
this field was 0 but it gained a new bit now. If we are to expose this
to arch-specific things, it may need some reservations. Anyway, that's
an optimisation that can be added subsequently.

-- 
Catalin