[PATCH bpf-next v2 04/11] bpf: Describe a BPF function to its trampoline as one u64 per slot

[Yonghong Song <yonghong.song@linux.dev>]

A trampoline on a BPF function, fentry, fexit or fsession attached
through attach_prog_fd, takes the function's arguments from where BPF
code left them. A call from one BPF function to another passes slot N
where the kernel convention passes its Nth u64 argument, in a register
or a stack slot, whatever the types of the arguments: the x86-64, arm64
and riscv JITs all place a stack argument that way. The model of the
function, distilled from its BTF, still carries those types, so each
arch places the arguments by its own rules for them. For a 16-byte
argument at an odd slot, such as s in

	u64 f(u64 a, struct { __int128 v; } s, u64 b)

the arm64 trampoline reads s from x2 and x3, where the caller left it in
x1 and x2, and b from x4. And an x86-64 trampoline puts a 16-byte
argument at slot 6 wholly on the stack, while BPF code splits it between
r9 and the stack.

Rewrite the model of a target program's function in
bpf_check_attach_target() as one u64 argument per argument slot.
Every convention places N u64 arguments in slot order, so all of the
above take them from where BPF code put them, and no arch has to know
that the function is a BPF one.

The model no longer gives the size and flags of each argument, so an
arch that sign-extends a narrow signed argument, or clears the unused
bytes of a narrow stack argument, for the program does not do that for a
BPF function any more: the program gets the eightbyte BPF code passed,
as it does on x86-64, and fexit hands it back unchanged.

Fixes: afdbb4cdd3d2 ("bpf, arm64: Place trampoline arguments by the arm64 calling convention")
Fixes: 4dd676ca5b7a ("bpf: Support by-value struct arguments up to 16 bytes")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2f190abe3448..c7af14a3f0e8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -22002,6 +22002,32 @@ static bool attach_uses_trampoline_retval(enum bpf_attach_type type)
 	}
 }
 
+/*
+ * BPF code passes argument slot N where the kernel convention passes its Nth
+ * u64 argument, so describe a BPF function to its trampoline as one u64 per
+ * slot, and every arch places its arguments right.
+ */
+static int flatten_bpf_func_model(struct bpf_verifier_log *log, const char *tname,
+				  struct btf_func_model *m)
+{
+	u32 i, nslots = 0;
+
+	for (i = 0; i < m->nr_args; i++)
+		nslots += btf_func_model_arg_slots(m, i);
+	if (nslots > MAX_BPF_FUNC_ARGS) {
+		bpf_log(log, "The function %s has %u argument slots. Too many.\n",
+			tname, nslots);
+		return -EINVAL;
+	}
+
+	for (i = 0; i < nslots; i++) {
+		m->arg_size[i] = sizeof(u64);
+		m->arg_flags[i] = 0;
+	}
+	m->nr_args = nslots;
+	return 0;
+}
+
 int bpf_check_attach_target(struct bpf_verifier_log *log,
 			    const struct bpf_prog *prog,
 			    const struct bpf_prog *tgt_prog,
@@ -22275,6 +22301,11 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 		ret = btf_distill_func_proto(log, btf, t, tname, &tgt_info->fmodel);
 		if (ret < 0)
 			return ret;
+		if (tgt_prog) {
+			ret = flatten_bpf_func_model(log, tname, &tgt_info->fmodel);
+			if (ret < 0)
+				return ret;
+		}
 
 		if (tgt_info->fmodel.ret_size > 8 &&
 		    attach_uses_trampoline_retval(prog->expected_attach_type)) {
-- 
2.53.0-Meta