bpf: don't infer PTR_TO_CTX for programs with unnamed context type

For program types that don't have named context type name (e.g., BPF
iterator programs or tracepoint programs), ctx_tname will be a non-NULL
empty string. For such programs it shouldn't be possible to have
PTR_TO_CTX argument for global subprogs based on type name alone.
arg:ctx tag is the only way to have PTR_TO_CTX passed into global
subprog for such program types.

Fix this loophole, which currently would assume PTR_TO_CTX whenever
user uses a pointer to anonymous struct as an argument to their global
subprogs. This happens in practice with the following (quite common, in
practice) approach:

typedef struct { /* anonymous */
    int x;
} my_type_t;

int my_subprog(my_type_t *arg) { ... }

User's intent is to have PTR_TO_MEM argument for `arg`, but verifier
will complain about expecting PTR_TO_CTX.

This fix also closes unintended s390x-specific KPROBE handling of
PTR_TO_CTX case. Selftest change is necessary to accommodate this.

Fixes: 91cc1a99740e ("bpf: Annotate context types")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240212233221.2575350-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 26dc0876e426..6ff0bd1a91d5 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5746,6 +5746,9 @@ again:
 		bpf_log(log, "Please fix kernel include/linux/bpf_types.h\n");
 		return false;
 	}
+	/* program types without named context types work only with arg:ctx tag */
+	if (ctx_tname[0] == '\0')
+		return false;
 	/* only compare that prog's ctx type name is the same as
 	 * kernel expects. No need to compare field by field.
 	 * It's ok for bpf prog to do: