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    SubjectRe: [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/

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