On Tue, Feb 22, 2022 at 08:51:06AM -0800, Kalesh Singh wrote: > From: Quentin Perret <qperret@xxxxxxxxxx> > > The asm entry code in the kernel uses a trick to check if VMAP'd stacks > have overflowed by aligning them at THREAD_SHIFT * 2 granularity and > checking the SP's THREAD_SHIFT bit. > > Protected KVM will soon make use of a similar trick to detect stack > overflows, so factor out the asm code in a re-usable macro. > > Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx> > [Kalesh - Resolve minor conflicts] > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx> > --- > arch/arm64/include/asm/assembler.h | 11 +++++++++++ > arch/arm64/kernel/entry.S | 7 +------ > 2 files changed, 12 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h > index e8bd0af0141c..ad40eb0eee83 100644 > --- a/arch/arm64/include/asm/assembler.h > +++ b/arch/arm64/include/asm/assembler.h > @@ -850,4 +850,15 @@ alternative_endif > > #endif /* GNU_PROPERTY_AARCH64_FEATURE_1_DEFAULT */ > > +/* > + * Test whether the SP has overflowed, without corrupting a GPR. > + */ > +.macro test_sp_overflow shift, label > + add sp, sp, x0 // sp' = sp + x0 > + sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp > + tbnz x0, #\shift, \label > + sub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0 > + sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = sp > +.endm I'm a little unhappy about factoring this out, since it's not really self-contained and leaves sp and x0 partially-swapped when it branches to the label. You can't really make that clear with comments on the macro, and you need comments at each use-sire, so I'd ratehr we just open-coded a copy of this. > + > #endif /* __ASM_ASSEMBLER_H */ > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S > index 772ec2ecf488..ce99ee30c77e 100644 > --- a/arch/arm64/kernel/entry.S > +++ b/arch/arm64/kernel/entry.S > @@ -53,15 +53,10 @@ alternative_else_nop_endif > sub sp, sp, #PT_REGS_SIZE > #ifdef CONFIG_VMAP_STACK > /* > - * Test whether the SP has overflowed, without corrupting a GPR. > * Task and IRQ stacks are aligned so that SP & (1 << THREAD_SHIFT) > * should always be zero. > */ > - add sp, sp, x0 // sp' = sp + x0 > - sub x0, sp, x0 // x0' = sp' - x0 = (sp + x0) - x0 = sp > - tbnz x0, #THREAD_SHIFT, 0f > - sub x0, sp, x0 // x0'' = sp' - x0' = (sp + x0) - sp = x0 > - sub sp, sp, x0 // sp'' = sp' - x0 = (sp + x0) - x0 = sp > + test_sp_overflow THREAD_SHIFT, 0f > b el\el\ht\()_\regsize\()_\label > > 0: Further to my comment above, immediately after this we have: /* Stash the original SP (minus PT_REGS_SIZE) in tpidr_el0. */ msr tpidr_el0, x0 /* Recover the original x0 value and stash it in tpidrro_el0 */ sub x0, sp, x0 msr tpidrro_el0, x0 ... which is really surprising with the `test_sp_overflow` macro because it's not clear that modifies x0 and sp in this way. Thanks, Mark. ... > -- > 2.35.1.473.g83b2b277ed-goog > _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
