Support -mstack-protector-guard=sysreg for riscv

[kees]

Bugzilla Link	47341
Version	trunk
OS	Linux
Blocks	#4440
CC	@efriedma-quic,@nickdesaulniers,@zygoloid,@stephenhines

Extended Description

The Linux kernel uses this for having a per-thread stack canary. Implementing this for both arm64 and riscv is desired.

For example, gcc's arm64 support is used like this in the kernel:

-mstack-protector-guard=sysreg
-mstack-protector-guard-reg=sp_el0
-mstack-protector-guard-offset=0

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a1213fa7432778b71a1c0166bf56660a3aab030

Here are discussions on the start of riscv support:

https://lore.kernel.org/lkml/CAJF2gTTr_ENhGRmjqNbGBhEc8y4D3YSqAResvwHuB1ykemKTUw@mail.gmail.com/

[nickdesaulniers]

SelectionDAGBuilder::visitSPDescriptorParent is probably the most relevant method for this. I'm not sure yet how best to modify getLoadStackGuard in order to change this from a load of a global Value to an aarch64 specific mrs instruction.

Here's a basic test case demonstrating the differences between these 3 new flags, vs without them (but still with -fstack-protector-strong).

https://godbolt.org/z/oEc1fvMPo

[nickdesaulniers]

Ah looks like I can just add custom expansion for AArch64::LOAD_STACK_GUARD in AArch64InstrInfo::expandPostRAPseudo() and that seems to work.

[nickdesaulniers]

aarch64: https://reviews.llvm.org/D100919

[nickdesaulniers]

https://reviews.llvm.org/rG0f417789192e74f9d2fad0f6aee4efc394257176 has landed for aarch64 with the caveats that we only support sp_el0 for valid sysreg, and we don't try too hard to support arbitrary int32_t's for offsets.

[nickdesaulniers]

For the Linux kernel, it looks like risc-v uses

-mstack-protector-guard=tls
-mstack-protector-guard-reg=tp
-mstack-protector-guard-offset=

It looks like ppc uses:

-mstack-protector-guard=tls
-mstack-protector-guard-reg={r13|r2}
-mstack-protector-guard-offset=

x86 uses:

-mstack-protector-guard-reg=fs
-mstack-protector-guard-symbol=__stack_chk_guard
-mstack-protector-guard=global

See #48553 for -mstack-protector-guard-symbol=.

[nickdesaulniers]

mentioned in issue llvm/llvm-bugzilla-archive#49209

[llvmbot]

@llvm/issue-subscribers-clang-driver

[nickdesaulniers]

cc @asb

[xgupta]

I thought to look at RISCV but it seems they don't have basic stack guard protection. so that needs to be implemented first (like powerpc or sparc) and then customization of stack protector guard.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v