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18:48:26 +0100 Date: Thu, 22 Oct 2020 18:48:26 +0100 From: Russell King - ARM Linux admin To: Ard Biesheuvel Subject: Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error Message-ID: <20201022174826.GS1551@shell.armlinux.org.uk> References: <202010211637.7CFD8435@keescook> <773fbdb0-5fc4-ab39-e72d-89845faa4c6d@gmail.com> <202010212028.32E8A5EF9B@keescook> <1d2e2b5d-3035-238c-d2ca-14c0c209a6a1@gmail.com> <20201022161118.GP1551@shell.armlinux.org.uk> <20201022162334.GQ1551@shell.armlinux.org.uk> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201022_134832_265579_2C494B5E X-CRM114-Status: GOOD ( 45.17 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kees Cook , Nick Desaulniers , Linux 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<773fbdb0-5fc4-ab39-e72d-89845faa4c6d@gmail.com> <202010212028.32E8A5EF9B@keescook> <1d2e2b5d-3035-238c-d2ca-14c0c209a6a1@gmail.com> <20201022161118.GP1551@shell.armlinux.org.uk> <20201022162334.GQ1551@shell.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: Russell King - ARM Linux admin Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 22, 2020 at 06:33:17PM +0200, Ard Biesheuvel wrote: > On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin > wrote: > > > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote: > > > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin > > > wrote: > > > > > > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote: > > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko wrote: > > > > > > > > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет: > > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook wrote: > > > > > > >> > > > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote: > > > > > > >>> 22.10.2020 02:40, Kees Cook пишет: > > > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote: > > > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative > > > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation > > > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'" > > > > > > >>>>> linker error. Let's use long jump in order to fix the issue. > > > > > > >>>> > > > > > > >>>> Eek. Is this with gcc or clang? > > > > > > >>> > > > > > > >>> GCC 9.3.0 > > > > > > >>> > > > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections") > > > > > > >>>> > > > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ? > > > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer, > > > > > > >>>> though I thought I had chosen the correct position. > > > > > > >>> > > > > > > >>> I re-checked that the fixes tag is correct. > > > > > > >>> > > > > > > >>>>> Signed-off-by: Dmitry Osipenko > > > > > > >>>>> --- > > > > > > >>>>> arch/arm/vfp/vfphw.S | 3 ++- > > > > > > >>>>> 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > >>>>> > > > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S > > > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644 > > > > > > >>>>> --- a/arch/arm/vfp/vfphw.S > > > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S > > > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry) > > > > > > >>>>> ldr r3, [sp, #S_PSR] @ Neither lazy restore nor FP exceptions > > > > > > >>>>> and r3, r3, #MODE_MASK @ are supported in kernel mode > > > > > > >>>>> teq r3, #USR_MODE > > > > > > >>>>> - bne vfp_kmode_exception @ Returns through lr > > > > > > >>>>> + ldr r1, =vfp_kmode_exception > > > > > > >>>>> + bxne r1 @ Returns through lr > > > > > > >>>>> > > > > > > >>>>> VFPFMRX r1, FPEXC @ Is the VFP enabled? > > > > > > >>>>> DBGSTR1 "fpexc %08x", r1 > > > > > > >>>> > > > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs > > > > > > >>>> moving? > > > > > > >>>> > > > > > > >>> > > > > > > >>> I don't know where it needs to be moved. Please feel free to make a > > > > > > >>> patch if you have a better idea, I'll be glad to test it. > > > > > > >> > > > > > > >> I might have just been distracted by the common "vfp" prefix. It's > > > > > > >> possible that the text section shuffling just ended up being very large, > > > > > > >> so probably this patch is right then! > > > > > > >> > > > > > > > > > > > > > > I already sent a fix for this issue: > > > > > > > > > > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1 > > > > > > > > > > > > > > > > > > > The offending commit contains stable tag, so I assume that fixes tag is > > > > > > mandatory. Yours patch misses the fixes tag. > > > > > > > > > > Russell, mind adding that? Or would you like me to update the patch in > > > > > the patch system? > > > > > > > > Rather than adding the IT, I'm suggesting that we solve it a different > > > > way - ensuring that the two bits of code are co-located. There's no > > > > reason for them to be separated, and the assembly code entry point is > > > > already called indirectly. > > > > > > > > The problem is the assembly ends up in the .text section which ends up > > > > at the start of the binary, but depending on the compiler, functions > > > > in .c files end up in their own sections. It would be good if, as > > > > Dmitry has shown that it is indeed possible, to have them co-located. > > > > > > Why is that better? I provided a minimal fix which has zero impact on > > > ARM builds, and minimal impact on Thumb2 builds, given that it retains > > > the exact same semantics as before, but using a different opcode. > > > > I think you just described the reason there. Why should we force > > everything to use a different opcode when a short jump _should_ > > suffice? > > > > Why should a short jump suffice? The call is to vfp_kmode_exception(), > which we only call in exceptional cases. Why would we want to keep > that in close proximity? You're thinking about it in terms of what happens when the branch is taken, rather than also considering that this code path is also traversed for _every_ single time that we enter the support code not just for kernel mode. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!