libbpf: Parse usdt args without offset on x86 (e.g. 8@(%rsp))

Parse USDT arguments like "8@(%rsp)" on x86. These are emmited by
SystemTap. The argument syntax is similar to the existing "memory
dereference case" but the offset left out as it's zero (i.e. read
the value from the address in the register). We treat it the same
as the the "memory dereference case", but set the offset to 0.

I've tested that this fixes the "unrecognized arg #N spec: 8@(%rsp).."
error I've run into when attaching to a probe with such an argument.
Attaching and reading the correct argument values works.

Something similar might be needed for the other supported
architectures.

  [0] Closes: https://github.com/libbpf/libbpf/issues/559Signed-off-by: Timo Hunziker <timo.hunziker@gmx.ch>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221203123746.2160-1-timo.hunziker@eclipso.ch

tools/lib/bpf/usdt.c

@@ -1233,6 +1233,14 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec
 		if (reg_off < 0)
 			return reg_off;
 		arg->reg_off = reg_off;
+	} else if (sscanf(arg_str, " %d @ ( %%%15[^)] ) %n", &arg_sz, reg_name, &len) == 2) {
+		/* Memory dereference case without offset, e.g., 8@(%rsp) */
+		arg->arg_type = USDT_ARG_REG_DEREF;
+		arg->val_off = 0;
+		reg_off = calc_pt_regs_off(reg_name);
+		if (reg_off < 0)
+			return reg_off;
+		arg->reg_off = reg_off;
 	} else if (sscanf(arg_str, " %d @ %%%15s %n", &arg_sz, reg_name, &len) == 2) {
 		/* Register read case, e.g., -4@%eax */
 		arg->arg_type = USDT_ARG_REG;