Re: [PATCH v4 bpf-next] bpf: Explicitly zero-extend R0 after 32-bit cmpxchg

On Wed, 24 Feb 2021 at 06:48, Martin KaFai Lau [off-list ref] wrote:

On Tue, Feb 23, 2021 at 03:08:45PM +0000, Brendan Jackman wrote:
[ ... ]

quoted

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 0ae015ad1e05..dcf18612841b 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c

@@ -2342,6 +2342,10 @@ bool __weak bpf_helper_changes_pkt_data(void *func)
 /* Return TRUE if the JIT backend wants verifier to enable sub-register usage
  * analysis code and wants explicit zero extension inserted by verifier.
  * Otherwise, return FALSE.
+ *
+ * The verifier inserts an explicit zero extension after BPF_CMPXCHGs even if
+ * you don't override this. JITs that don't want these extra insns can detect
+ * them using insn_is_zext.
  */
 bool __weak bpf_jit_needs_zext(void)
 {

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3d34ba492d46..ec1cbd565140 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -11061,8 +11061,16 @@ static int opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
                       */
                      if (WARN_ON(!(insn.imm & BPF_FETCH)))
                              return -EINVAL;
-                     load_reg = insn.imm == BPF_CMPXCHG ? BPF_REG_0
-                                                        : insn.src_reg;
+                     /* There should already be a zero-extension inserted after BPF_CMPXCHG. */
+                     if (insn.imm == BPF_CMPXCHG) {
+                             struct bpf_insn *next = &insns[adj_idx + 1];
+
+                             if (WARN_ON(!insn_is_zext(next) || next->dst_reg != insn.src_reg))
+                                     return -EINVAL;
+                             continue;

This is to avoid zext_patch again for the JITs with
bpf_jit_needs_zext() == true.

IIUC, at this point, aux[adj_idx].zext_dst == true which
means that the check_atomic() has already marked the
reg0->subreg_def properly.

That's right... sorry I'm not sure if you're implying something here
or just checking understanding?

quoted

+                     }
+
+                     load_reg = insn.src_reg;
              } else {
                      load_reg = insn.dst_reg;
              }

@@ -11666,6 +11674,27 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
                      continue;
              }

+             /* BPF_CMPXCHG always loads a value into R0, therefore always
+              * zero-extends. However some archs' equivalent instruction only
+              * does this load when the comparison is successful. So here we
+              * add a BPF_ZEXT_REG after every 32-bit CMPXCHG, so that such
+              * archs' JITs don't need to deal with the issue. Archs that
+              * don't face this issue may use insn_is_zext to detect and skip
+              * the added instruction.
+              */
+             if (insn->code == (BPF_STX | BPF_W | BPF_ATOMIC) && insn->imm == BPF_CMPXCHG) {
+                     struct bpf_insn zext_patch[2] = { *insn, BPF_ZEXT_REG(BPF_REG_0) };

Then should this zext_patch only be done for "!bpf_jit_needs_zext()"
such that the above change in opt_subreg_zext_lo32_rnd_hi32()
becomes unnecessary?

Yep that would work but I IMO it would be a more fragile expression of
the logic: instead of directly checking whether something was done
we'd be looking at a proxy for another part of the system's behaviour.
I don't think it would win us anything in terms of clarity either?

Thanks for taking a look!