From: Catalin Marinas <catalin.marinas@arm.com>
To: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Nick Desaulniers <ndesaulniers@google.com>,
LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64: Allow disabling of the compat vDSO
Date: Thu, 26 Sep 2019 08:47:18 +0100 [thread overview]
Message-ID: <20190926074717.GA26802@iMac.local> (raw)
In-Reply-To: <a7e06b86-facd-21de-c47c-246d0da8d80d@arm.com>
On Thu, Sep 26, 2019 at 01:06:50AM +0100, Vincenzo Frascino wrote:
> On 9/25/19 6:08 PM, Catalin Marinas wrote:
> > On Wed, Sep 25, 2019 at 09:53:16AM -0700, Nick Desaulniers wrote:
> >> On Wed, Sep 25, 2019 at 6:09 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> >>> Suggestions for future improvements of the compat vDSO handling:
> >>>
> >>> - replace the CROSS_COMPILE_COMPAT prefix with a full COMPATCC; maybe
> >>> check that it indeed produces 32-bit code
>
> CROSS_COMPILE_COMPAT is called like this for symmetry with CROSS_COMPILE.
Actually, what gets in the way is the CONFIG_CROSS_COMPILE_COMPAT_VDSO.
We can keep CROSS_COMPILE_COMPAT together with COMPATCC initialised to
$(CROSS_COMPILE_COMPAT)gcc. When we will be able to build the compat
vDSO with clang, we just pass COMPATCC=clang on the make line and the
kernel Makefile will figure out the --target option from
CROSS_COMPILE_COMPAT (see how CLANG_FLAGS is handled).
If we stick only to env variables or make cmd line (without Kconfig) for
the compiler name, we can add a COMPATCC_IS_CLANG in the Kconfig
directly and simply not allow the enabling the COMPAT_VDSO if we don't
have the right compiler. This saves us warnings during build.
> >>> - check whether COMPATCC is clang or not rather than CC_IS_CLANG, which
> >>> only checks the native compiler
> >>
> >> When cross compiling, IIUC CC_IS_CLANG is referring to CC which is the
> >> cross compiler, which is different than HOSTCC which is the host
> >> compiler. HOSTCC is used mostly for things in scripts/ while CC is
> >> used to compile a majority of the kernel in a cross compile.
> >
> > We need the third compiler here for the compat vDSO (at least with gcc),
> > COMPATCC. I'm tempted to just drop the CONFIG_CROSS_COMPILE_COMPAT_VDSO
> > altogether and only rely on a COMPATCC. This way we can add
> > COMPATCC_IS_CLANG etc. in the Kconfig checks directly.
> >
> > If clang can build both 32 and 64-bit with the same binary (just
> > different options), we could maybe have COMPATCC default to CC and add a
> > check on whether COMPATCC generates 32-bit binaries.
>
> clang requires the --target option for specifying the 32bit triple.
> Basically $(TRIPLE)-gcc is equivalent to gcc --target $(TRIPLE).
> We need a configuration option to encode this.
Since we don't have a CONFIG_* option for the cross-compiler prefix, we
shouldn't have one for the compat compiler either. If you want to build
the compat vDSO with clang, just pass COMPATCC=clang together with
CROSS_COMPILE_COMPAT. We can add Kconfig checks to actually verify that
COMPATCC generates 32-bit binaries (e.g. COMPATCC_CAN_LINK32).
> >>> - clean up the headers includes; vDSO should not include kernel-only
> >>> headers that may even contain code patched at run-time
> >>
> >> This is a big one; Clang validates the inline asm constraints for
> >> extended inline assembly, GCC does not for dead code. So Clang chokes
> >> on the inclusion of arm64 headers using extended inline assembly when
> >> being compiled for arm-linux-gnueabi.
> >
> > Whether clang or gcc, I'd like this fixed anyway. At some point we may
> > inadvertently rely on some code which is patched at boot time for the
> > kernel code but not for the vDSO.
>
> Do we have any code of this kind in header files?
>
> The vDSO library uses only a subset of the headers (mainly Macros) hence all the
> unused symbols should be compiled out. Is your concern only theoretical or do
> you have an example on where this could be happening?
At the moment it's rather theoretical.
--
Catalin
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-09-26 7:47 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-25 13:09 [PATCH] arm64: Allow disabling of the compat vDSO Catalin Marinas
2019-09-25 16:53 ` Nick Desaulniers
2019-09-25 17:08 ` Catalin Marinas
2019-09-25 17:31 ` Nick Desaulniers
2019-09-25 23:35 ` Vincenzo Frascino
2019-09-26 0:06 ` Vincenzo Frascino
2019-09-26 7:47 ` Catalin Marinas [this message]
2019-09-26 15:51 ` Catalin Marinas
2019-09-26 16:40 ` Nick Desaulniers
2019-09-26 16:38 ` Nick Desaulniers
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190926074717.GA26802@iMac.local \
--to=catalin.marinas@arm.com \
--cc=ard.biesheuvel@linaro.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=ndesaulniers@google.com \
--cc=tglx@linutronix.de \
--cc=vincenzo.frascino@arm.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).