bpf: Specify access type of bpf_sysctl_get_name args

The second argument of bpf_sysctl_get_name() helper is a pointer to a
buffer that is being written to. However that isn't specify in the
prototype.

Until commit 37cce22dbd51a ("bpf: verifier: Refactor helper access
type tracking"), all helper accesses were considered as a possible
write access by the verifier, so no big harm was done. However, since
then, the verifier might make wrong asssumption about the content of
that address which might lead it to make faulty optimizations (such as
removing code that was wrongly labeled dead). This is what happens in
test_sysctl selftest to the tests related to sysctl_get_name.

Add MEM_WRITE flag the second argument of bpf_sysctl_get_name().

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250619140603.148942-2-jmarchan@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 9122c39870bff858a08f0d03207c6be33a49bf7d..f4885514f007b3da34e505aad899f5f2ec4eaba2 100644 (file)
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -2134,7 +2134,7 @@ static const struct bpf_func_proto bpf_sysctl_get_name_proto = {
        .gpl_only       = false,
        .ret_type       = RET_INTEGER,
        .arg1_type      = ARG_PTR_TO_CTX,
-       .arg2_type      = ARG_PTR_TO_MEM,
+       .arg2_type      = ARG_PTR_TO_MEM | MEM_WRITE,
        .arg3_type      = ARG_CONST_SIZE,
        .arg4_type      = ARG_ANYTHING,
 };