bpf: teach verifier actual bounds of bpf_get_smp_processor_id() result

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 <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Alexei Starovoitov <[email protected]>
kawasaki · Jul 12, 2023 · f42bcd1 · f42bcd1
1 parent 87e098e
commit f42bcd1
Showing 1 changed file with 26 additions and 11 deletions.
diff --git 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