bpf: Recognize btf_decl_tag("arg: Arena") as PTR_TO_ARENA.

In global bpf functions recognize btf_decl_tag("arg:arena") as PTR_TO_ARENA.

Note, when the verifier sees:

__weak void foo(struct bar *p)

it recognizes 'p' as PTR_TO_MEM and 'struct bar' has to be a struct with scalars.
Hence the only way to use arena pointers in global functions is to tag them with "arg:arena".

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/linux/bpf.h

···
 	 * on eBPF program stack
 	 */
 	ARG_PTR_TO_MEM,		/* pointer to valid memory (stack, packet, map value) */
+	ARG_PTR_TO_ARENA,
 
 	ARG_CONST_SIZE,		/* number of bytes accessed from memory */
 	ARG_CONST_SIZE_OR_ZERO,	/* number of bytes accessed from memory or 0 */

kernel/bpf/btf.c

···
 }
 
 enum btf_arg_tag {
-	ARG_TAG_CTX = 0x1,
-	ARG_TAG_NONNULL = 0x2,
-	ARG_TAG_TRUSTED = 0x4,
-	ARG_TAG_NULLABLE = 0x8,
+	ARG_TAG_CTX	 = BIT_ULL(0),
+	ARG_TAG_NONNULL  = BIT_ULL(1),
+	ARG_TAG_TRUSTED  = BIT_ULL(2),
+	ARG_TAG_NULLABLE = BIT_ULL(3),
+	ARG_TAG_ARENA	 = BIT_ULL(4),
 };
 
 /* Process BTF of a function to produce high-level expectation of function
···
 				tags |= ARG_TAG_NONNULL;
 			} else if (strcmp(tag, "nullable") == 0) {
 				tags |= ARG_TAG_NULLABLE;
+			} else if (strcmp(tag, "arena") == 0) {
+				tags |= ARG_TAG_ARENA;
 			} else {
 				bpf_log(log, "arg#%d has unsupported set of tags\n", i);
 				return -EOPNOTSUPP;
···
 			if (tags & ARG_TAG_NULLABLE)
 				sub->args[i].arg_type |= PTR_MAYBE_NULL;
 			sub->args[i].btf_id = kern_type_id;
+			continue;
+		}
+		if (tags & ARG_TAG_ARENA) {
+			if (tags & ~ARG_TAG_ARENA) {
+				bpf_log(log, "arg#%d arena cannot be combined with any other tags\n", i);
+				return -EINVAL;
+			}
+			sub->args[i].arg_type = ARG_PTR_TO_ARENA;
 			continue;
 		}
 		if (is_global) { /* generic user data pointer */

kernel/bpf/verifier.c

···
 				bpf_log(log, "arg#%d is expected to be non-NULL\n", i);
 				return -EINVAL;
 			}
+		} else if (base_type(arg->arg_type) == ARG_PTR_TO_ARENA) {
+			/*
+			 * Can pass any value and the kernel won't crash, but
+			 * only PTR_TO_ARENA or SCALAR make sense. Everything
+			 * else is a bug in the bpf program. Point it out to
+			 * the user at the verification time instead of
+			 * run-time debug nightmare.
+			 */
+			if (reg->type != PTR_TO_ARENA && reg->type != SCALAR_VALUE) {
+				bpf_log(log, "R%d is not a pointer to arena or scalar.\n", regno);
+				return -EINVAL;
+			}
 		} else if (arg->arg_type == (ARG_PTR_TO_DYNPTR | MEM_RDONLY)) {
 			ret = process_dynptr_func(env, regno, -1, arg->arg_type, 0);
 			if (ret)
···
 				reg->btf = bpf_get_btf_vmlinux(); /* can't fail at this point */
 				reg->btf_id = arg->btf_id;
 				reg->id = ++env->id_gen;
+			} else if (base_type(arg->arg_type) == ARG_PTR_TO_ARENA) {
+				/* caller can pass either PTR_TO_ARENA or SCALAR */
+				mark_reg_unknown(env, regs, i);
 			} else {
 				WARN_ONCE(1, "BUG: unhandled arg#%d type %d\n",
 					  i - BPF_REG_1, arg->arg_type);