bpf: Specify access type of bpf_sysctl_get_name args

JIRA: https://issues.redhat.com/browse/RHEL-93930

commit 2eb7648
Author: Jerome Marchand <jmarchan@redhat.com>
Date:   Thu Jun 19 16:06:02 2025 +0200

    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 37cce22 ("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>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/bpf/cgroup.c

@@ -2087,7 +2087,7 @@ 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,
 };