[PATCH bpf-next 0/6] bpf: verifier: stop emitting zext for LDX

Puranjay Mohan puranjay12 at gmail.com
Wed Sep 13 08:46:48 AEST 2023

All 64-bit architectures that support the BPF JIT do LDX + zero extension
with a single CPU instruction. Some 64-bit architectures like riscv64,
s390, mips64, etc. have bpf_jit_needs_zext() -> true. This means although
these architectures do LDX + zero extension with a single CPU instruction,
the verifier emits extra zero extension instructions after LDX | B/H/W.

After a discussion about this in [1], it was decided that the verifier
should not emit zext instructions for LDX and all JITs that can't do a LDX
+ zero extension with a single instruction should emit two instructions by
default for LDX.

All 32 bit JITs checked for ctx->prog->aux->verifier_zext and did not do
explicit zero extension after LDX if this is set by the verifier.

This patch series changes all applicable 32-bit JITs to always do a zero
extension after LDX without checking ctx->prog->aux->verifier_zext.

The last patch modifies the verifier to always mark the destination of LDX
as 64 bit which in turn stops the verifier from emitting zext after LDX.

These changes have not been tested because I don't have the hardware to do
so, I would request the JIT maintainers to help me test this. Especially,
the powerpc32 JTI where amount of code change is more.

[1] https://lore.kernel.org/all/CANk7y0j2f-gPgZwd+YfTL71-6wfvky+f=kBC_ccqsS0EHAysyA@mail.gmail.com/

Puranjay Mohan (6):
  bpf, riscv32: Always zero extend for LDX with B/W/H
  bpf, x86-32: Always zero extend for LDX with B/W/H
  bpf, parisc32: Always zero extend for LDX with B/W/H
  bpf, powerpc32: Always zero extend for LDX
  bpf, arm32: Always zero extend for LDX with B/H/W
  bpf, verifier: always mark destination of LDX as 64-bit

 arch/arm/net/bpf_jit_32.c         |  9 +++------
 arch/parisc/net/bpf_jit_comp32.c  |  9 +++------
 arch/powerpc/net/bpf_jit_comp32.c | 25 ++++++++-----------------
 arch/riscv/net/bpf_jit_comp32.c   |  9 +++------
 arch/x86/net/bpf_jit_comp32.c     |  2 --
 kernel/bpf/verifier.c             |  4 +---
 6 files changed, 18 insertions(+), 40 deletions(-)


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