[PATCH bpf-next 1/2] bpf: teach verifier actual bounds of bpf_get_smp_processor_id() result

[Andrii Nakryiko <andrii@xxxxxxxxxx>]

bpf_get_smp_processor_id() helper returns current CPU on which BPF
program runs. It can't return value that is bigger than maximum allowed
number of CPUs (minus one, due to zero indexing). Teach BPF verifier to
recognize that. This makes it possible to use bpf_get_smp_processor_id()
result to index into arrays without extra checks, as demonstrated in
subsequent selftests/bpf patch.

Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 kernel/bpf/verifier.c | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 11e54dd8b6dd..81a93eeac7a0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -25,6 +25,7 @@
 #include <linux/btf_ids.h>
 #include <linux/poison.h>
 #include <linux/module.h>
+#include <linux/cpumask.h>
 
 #include "disasm.h"
 
@@ -9116,19 +9117,33 @@ static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type,
 {
 	struct bpf_reg_state *ret_reg = &regs[BPF_REG_0];
 
-	if (ret_type != RET_INTEGER ||
-	    (func_id != BPF_FUNC_get_stack &&
-	     func_id != BPF_FUNC_get_task_stack &&
-	     func_id != BPF_FUNC_probe_read_str &&
-	     func_id != BPF_FUNC_probe_read_kernel_str &&
-	     func_id != BPF_FUNC_probe_read_user_str))
+	if (ret_type != RET_INTEGER)
 		return;
 
-	ret_reg->smax_value = meta->msize_max_value;
-	ret_reg->s32_max_value = meta->msize_max_value;
-	ret_reg->smin_value = -MAX_ERRNO;
-	ret_reg->s32_min_value = -MAX_ERRNO;
-	reg_bounds_sync(ret_reg);
+	switch (func_id) {
+	case BPF_FUNC_get_stack:
+	case BPF_FUNC_get_task_stack:
+	case BPF_FUNC_probe_read_str:
+	case BPF_FUNC_probe_read_kernel_str:
+	case BPF_FUNC_probe_read_user_str:
+		ret_reg->smax_value = meta->msize_max_value;
+		ret_reg->s32_max_value = meta->msize_max_value;
+		ret_reg->smin_value = -MAX_ERRNO;
+		ret_reg->s32_min_value = -MAX_ERRNO;
+		reg_bounds_sync(ret_reg);
+		break;
+	case BPF_FUNC_get_smp_processor_id:
+		ret_reg->umax_value = nr_cpu_ids - 1;
+		ret_reg->u32_max_value = nr_cpu_ids - 1;
+		ret_reg->smax_value = nr_cpu_ids - 1;
+		ret_reg->s32_max_value = nr_cpu_ids - 1;
+		ret_reg->umin_value = 0;
+		ret_reg->u32_min_value = 0;
+		ret_reg->smin_value = 0;
+		ret_reg->s32_min_value = 0;
+		reg_bounds_sync(ret_reg);
+		break;
+	}
 }
 
 static int
-- 
2.34.1