Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 29 Oct 2020 10:56:22 +0100 | Subject | Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error |
| |
On Mon, 26 Oct 2020 at 09:58, Ard Biesheuvel <ardb@kernel.org> wrote: > > On Thu, 22 Oct 2020 at 19:59, Ard Biesheuvel <ardb@kernel.org> wrote: > > > > On Thu, 22 Oct 2020 at 19:48, Russell King - ARM Linux admin > > <linux@armlinux.org.uk> wrote: > > > > > > 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 > > > > <linux@armlinux.org.uk> 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 > > > > > > <linux@armlinux.org.uk> wrote: > > > > > > > > > > > > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote: > > > > > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote: > > > > > > > > > > > > > > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет: > > > > > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> 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 <digetx@gmail.com> > > > > > > > > > >>>>> --- > > > > > > > > > >>>>> 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. > > > > > > > True. If 2 bytes of additional opcode are the concern here, we can > > change the current sequence > > > > 6: f093 0f10 teq r3, #16 > > a: f47f affe bne.w 0 <vfp_kmode_exception> > > > > to > > > > 6: 2b10 cmp r3, #16 > > 8: bf18 it ne > > a: f7ff bffe bne.w 0 <vfp_kmode_exception> > > > > which takes up the exact same space. > > BTW this code path looks slightly broken for Thumb-2 in any case: if a > FP exception is taken in kernel mode on a Thumb2 kernel, we enter the > emulation sequence via call_fpe, which will use the wrong set of > value/mask pairs to match the opcode. The minimal fix is to move the > call_fpe label to the right place, but I think it might be better to > move the check for a FP exception in kernel mode to the handling of > __und_svc.
Do we have a resolution here? This is causing breakage in kernelci
https://kernelci.org/build/id/5f9a834c5ed3c05dd538101b/
| |