| Message ID | 1429613983-22739-3-git-send-email-ard.biesheuvel@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, Apr 21, 2015 at 11:59:43AM +0100, Ard Biesheuvel wrote: > Commit 6ebbf2ce437b (ARM: convert all "mov.* pc, reg" to "bx reg" > for ARMv6+) replaced all occurrences of 'mov pc, <reg>' with the > 'ret' macro. However, this macro only emits the 'bx' instruction > when used with the 'lr' register, but still uses 'mov pc, <reg>' > for everything else. > > Since ARM/Thumb2 interworking is allowed in the static kernel > (i.e., inside vmlinux), this is potentially unsafe, since the > Thumb mov instruction T1 variant that allows PC as the target will > not switch modes based on the Thumb bit > > So when building a Thumb2 kernel, emit the 'bx' instruction for > all registers, not just the 'lr' register. > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> > --- > arch/arm/include/asm/assembler.h | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h > index 186270b3e194..1c057f923258 100644 > --- a/arch/arm/include/asm/assembler.h > +++ b/arch/arm/include/asm/assembler.h > @@ -434,6 +434,8 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) > .macro ret\c, reg > #if __LINUX_ARM_ARCH__ < 6 > mov\c pc, \reg > +#elif defined(CONFIG_THUMB2_KERNEL) > + bx\c \reg > #else In v7, !THUMB_KERNEL builds, we now do this: > .ifeqs "\reg", "lr" > bx\c \reg Why? A !THUMB2 kernel should not contain any Thumb code in general. Cheers ---Dave
On 21 April 2015 at 13:15, Dave P Martin <Dave.Martin@arm.com> wrote: > On Tue, Apr 21, 2015 at 11:59:43AM +0100, Ard Biesheuvel wrote: >> Commit 6ebbf2ce437b (ARM: convert all "mov.* pc, reg" to "bx reg" >> for ARMv6+) replaced all occurrences of 'mov pc, <reg>' with the >> 'ret' macro. However, this macro only emits the 'bx' instruction >> when used with the 'lr' register, but still uses 'mov pc, <reg>' >> for everything else. >> >> Since ARM/Thumb2 interworking is allowed in the static kernel >> (i.e., inside vmlinux), this is potentially unsafe, since the >> Thumb mov instruction T1 variant that allows PC as the target will >> not switch modes based on the Thumb bit >> >> So when building a Thumb2 kernel, emit the 'bx' instruction for >> all registers, not just the 'lr' register. >> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> >> --- >> arch/arm/include/asm/assembler.h | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h >> index 186270b3e194..1c057f923258 100644 >> --- a/arch/arm/include/asm/assembler.h >> +++ b/arch/arm/include/asm/assembler.h >> @@ -434,6 +434,8 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) >> .macro ret\c, reg >> #if __LINUX_ARM_ARCH__ < 6 >> mov\c pc, \reg >> +#elif defined(CONFIG_THUMB2_KERNEL) >> + bx\c \reg >> #else > > In v7, !THUMB_KERNEL builds, we now do this: > >> .ifeqs "\reg", "lr" >> bx\c \reg > > Why? A !THUMB2 kernel should not contain any Thumb code in general. > AFAIK 'bx lr' is the recommended return sequence, and using anything else may interfere with the return address prediction or whatever it is called.
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h index 186270b3e194..1c057f923258 100644 --- a/arch/arm/include/asm/assembler.h +++ b/arch/arm/include/asm/assembler.h @@ -434,6 +434,8 @@ THUMB( orr \reg , \reg , #PSR_T_BIT ) .macro ret\c, reg #if __LINUX_ARM_ARCH__ < 6 mov\c pc, \reg +#elif defined(CONFIG_THUMB2_KERNEL) + bx\c \reg #else .ifeqs "\reg", "lr" bx\c \reg
Commit 6ebbf2ce437b (ARM: convert all "mov.* pc, reg" to "bx reg" for ARMv6+) replaced all occurrences of 'mov pc, <reg>' with the 'ret' macro. However, this macro only emits the 'bx' instruction when used with the 'lr' register, but still uses 'mov pc, <reg>' for everything else. Since ARM/Thumb2 interworking is allowed in the static kernel (i.e., inside vmlinux), this is potentially unsafe, since the Thumb mov instruction T1 variant that allows PC as the target will not switch modes based on the Thumb bit So when building a Thumb2 kernel, emit the 'bx' instruction for all registers, not just the 'lr' register. Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> --- arch/arm/include/asm/assembler.h | 2 ++ 1 file changed, 2 insertions(+)