[PATCH bpf v3 2/5] bpf: Fix NULL-ptr-deref when showing a void BTF type

[Jiayuan Chen <jiayuan.chen@linux.dev>]

btf_modifier_show() resolves the modifier and then calls
btf_type_ops(t)->show() unconditionally. For the void type (type_id 0,
BTF_KIND_UNKN) kind_ops[] has no entry, so ->show is NULL.

The map dump path cannot reach a void type (a map key/value must have a
size and void has none), but bpf_snprintf_btf() takes a type_id straight
from the BPF program, and a "const void" (a modifier that resolves to
void, present in the vmlinux BTF) NULL-derefs there:

KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
RIP: 0010:btf_modifier_show (kernel/bpf/btf.c:2914)
Call Trace:
 <TASK>
 btf_type_show (kernel/bpf/btf.c:8251)
 btf_type_snprintf_show (kernel/bpf/btf.c:8321)
 bpf_snprintf_btf (kernel/trace/bpf_trace.c:1047)
 bpf_prog_test_run_raw_tp (net/bpf/test_run.c:829)
 __sys_bpf (kernel/bpf/syscall.c:4804)
 do_syscall_64 (arch/x86/entry/syscall_64.c:94)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
 </TASK>

void has no size, so there is nothing to render - not even a byte length
to fall back to dumping as raw hex. btf_df_show() is already the ->show
for the other kinds that carry no value to print - FWD, FUNC, FUNC_PROTO,
FLOAT and DECL_TAG - and emits an "<unsupported kind:N>" placeholder;
void belongs to the same group and only lacks a show op because it has
no kind_ops[] entry at all. Fall back to btf_df_show() in
btf_modifier_show() when the resolved type has no show op, so a void type
prints that placeholder instead of crashing; bpf_snprintf_btf() then
returns the length as usual.

Fixes: c4d0bfb45068 ("bpf: Add bpf_snprintf_btf helper")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 kernel/bpf/btf.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 91b8ce77f699..6e267c4c4e1f 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2911,7 +2911,15 @@ static void btf_modifier_show(const struct btf *btf,
 	else
 		t = btf_type_skip_modifiers(btf, type_id, NULL);
 
-	btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
+	/*
+	 * A modifier can resolve to the void type (e.g. "const void"), which
+	 * has no show op (kind_ops[BTF_KIND_UNKN] is NULL). Print a placeholder
+	 * instead of dereferencing NULL.
+	 */
+	if (!btf_type_ops(t))
+		btf_df_show(btf, t, type_id, data, bits_offset, show);
+	else
+		btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
 }
 
 static void btf_var_show(const struct btf *btf, const struct btf_type *t,
-- 
2.43.0