bpf: Add support for custom exception callbacks

By default, the subprog generated by the verifier to handle a thrown
exception hardcodes a return value of 0. To allow user-defined logic
and modification of the return value when an exception is thrown,
introduce the 'exception_callback:' declaration tag, which marks a
callback as the default exception handler for the program.

The format of the declaration tag is 'exception_callback:<value>', where
<value> is the name of the exception callback. Each main program can be
tagged using this BTF declaratiion tag to associate it with an exception
callback. In case the tag is absent, the default callback is used.

As such, the exception callback cannot be modified at runtime, only set
during verification.

Allowing modification of the callback for the current program execution
at runtime leads to issues when the programs begin to nest, as any
per-CPU state maintaing this information will have to be saved and
restored. We don't want it to stay in bpf_prog_aux as this takes a
global effect for all programs. An alternative solution is spilling
the callback pointer at a known location on the program stack on entry,
and then passing this location to bpf_throw as a parameter.

However, since exceptions are geared more towards a use case where they
are ideally never invoked, optimizing for this use case and adding to
the complexity has diminishing returns.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230912233214.1518551-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 21 insertions, 8 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 187b57276fec..f93e835d90af 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3310,10 +3310,10 @@ static int btf_find_kptr(const struct btf *btf, const struct btf_type *t,
 	return BTF_FIELD_FOUND;
 }
 
-static const char *btf_find_decl_tag_value(const struct btf *btf,
-					   const struct btf_type *pt,
-					   int comp_idx, const char *tag_key)
+const char *btf_find_decl_tag_value(const struct btf *btf, const struct btf_type *pt,
+				    int comp_idx, const char *tag_key)
 {
+	const char *value = NULL;
 	int i;
 
 	for (i = 1; i < btf_nr_types(btf); i++) {
@@ -3327,9 +3327,14 @@ static const char *btf_find_decl_tag_value(const struct btf *btf,
 			continue;
 		if (strncmp(__btf_name_by_offset(btf, t->name_off), tag_key, len))
 			continue;
-		return __btf_name_by_offset(btf, t->name_off) + len;
+		/* Prevent duplicate entries for same type */
+		if (value)
+			return ERR_PTR(-EEXIST);
+		value = __btf_name_by_offset(btf, t->name_off) + len;
 	}
-	return NULL;
+	if (!value)
+		return ERR_PTR(-ENOENT);
+	return value;
 }
 
 static int
@@ -3347,7 +3352,7 @@ btf_find_graph_root(const struct btf *btf, const struct btf_type *pt,
 	if (t->size != sz)
 		return BTF_FIELD_IGNORE;
 	value_type = btf_find_decl_tag_value(btf, pt, comp_idx, "contains:");
-	if (!value_type)
+	if (IS_ERR(value_type))
 		return -EINVAL;
 	node_field_name = strstr(value_type, ":");
 	if (!node_field_name)
@@ -6954,7 +6959,7 @@ int btf_check_subprog_call(struct bpf_verifier_env *env, int subprog,
  * (either PTR_TO_CTX or SCALAR_VALUE).
  */
 int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
-			  struct bpf_reg_state *regs)
+			  struct bpf_reg_state *regs, bool is_ex_cb)
 {
 	struct bpf_verifier_log *log = &env->log;
 	struct bpf_prog *prog = env->prog;
@@ -7011,7 +7016,7 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
 			tname, nargs, MAX_BPF_FUNC_REG_ARGS);
 		return -EINVAL;
 	}
-	/* check that function returns int */
+	/* check that function returns int, exception cb also requires this */
 	t = btf_type_by_id(btf, t->type);
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);
@@ -7060,6 +7065,14 @@ int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
 			i, btf_type_str(t), tname);
 		return -EINVAL;
 	}
+	/* We have already ensured that the callback returns an integer, just
+	 * like all global subprogs. We need to determine it only has a single
+	 * scalar argument.
+	 */
+	if (is_ex_cb && (nargs != 1 || regs[BPF_REG_1].type != SCALAR_VALUE)) {
+		bpf_log(log, "exception cb only supports single integer argument\n");
+		return -EINVAL;
+	}
 	return 0;
 }