libbpf: USDT arm arg parsing support

Parsing of USDT arguments is architecture-specific; on arm it is
relatively easy since registers used are r[0-10], fp, ip, sp, lr,
pc. Format is slightly different compared to aarch64; forms are

- "size @ [ reg, #offset ]" for dereferences, for example
  "-8 @ [ sp, #76 ]" ; " -4 @ [ sp ]"
- "size @ reg" for register values; for example
  "-4@r0"
- "size @ #value" for raw values; for example
  "-8@#1"

Add support for parsing USDT arguments for ARM architecture.

To test the above changes QEMU's virt[1] board with cortex-a15
CPU was used. libbpf-bootstrap's usdt example[2] was modified to attach
to a test program with DTRACE_PROBE1/2/3/4... probes to test different
combinations.

[1] https://www.qemu.org/docs/master/system/arm/virt.html
[2] https://github.com/libbpf/libbpf-bootstrap/blob/master/examples/c/usdt.bpf.c

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230307120440.25941-3-puranjay12@gmail.com

tools/lib/bpf/usdt.c

@@ -1465,6 +1465,86 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec
 	return len;
 }
 
+#elif defined(__arm__)
+
+static int calc_pt_regs_off(const char *reg_name)
+{
+	static struct {
+		const char *name;
+		size_t pt_regs_off;
+	} reg_map[] = {
+		{ "r0", offsetof(struct pt_regs, uregs[0]) },
+		{ "r1", offsetof(struct pt_regs, uregs[1]) },
+		{ "r2", offsetof(struct pt_regs, uregs[2]) },
+		{ "r3", offsetof(struct pt_regs, uregs[3]) },
+		{ "r4", offsetof(struct pt_regs, uregs[4]) },
+		{ "r5", offsetof(struct pt_regs, uregs[5]) },
+		{ "r6", offsetof(struct pt_regs, uregs[6]) },
+		{ "r7", offsetof(struct pt_regs, uregs[7]) },
+		{ "r8", offsetof(struct pt_regs, uregs[8]) },
+		{ "r9", offsetof(struct pt_regs, uregs[9]) },
+		{ "r10", offsetof(struct pt_regs, uregs[10]) },
+		{ "fp", offsetof(struct pt_regs, uregs[11]) },
+		{ "ip", offsetof(struct pt_regs, uregs[12]) },
+		{ "sp", offsetof(struct pt_regs, uregs[13]) },
+		{ "lr", offsetof(struct pt_regs, uregs[14]) },
+		{ "pc", offsetof(struct pt_regs, uregs[15]) },
+	};
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(reg_map); i++) {
+		if (strcmp(reg_name, reg_map[i].name) == 0)
+			return reg_map[i].pt_regs_off;
+	}
+
+	pr_warn("usdt: unrecognized register '%s'\n", reg_name);
+	return -ENOENT;
+}
+
+static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg, int *arg_sz)
+{
+	char reg_name[16];
+	int len, reg_off;
+	long off;
+
+	if (sscanf(arg_str, " %d @ \[ %15[a-z0-9] , #%ld ] %n",
+		   arg_sz, reg_name, &off, &len) == 3) {
+		/* Memory dereference case, e.g., -4@[fp, #96] */
+		arg->arg_type = USDT_ARG_REG_DEREF;
+		arg->val_off = off;
+		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 @ \[ %15[a-z0-9] ] %n", arg_sz, reg_name, &len) == 2) {
+		/* Memory dereference case, e.g., -4@[sp] */
+		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 @ #%ld %n", arg_sz, &off, &len) == 2) {
+		/* Constant value case, e.g., 4@#5 */
+		arg->arg_type = USDT_ARG_CONST;
+		arg->val_off = off;
+		arg->reg_off = 0;
+	} else if (sscanf(arg_str, " %d @ %15[a-z0-9] %n", arg_sz, reg_name, &len) == 2) {
+		/* Register read case, e.g., -8@r4 */
+		arg->arg_type = USDT_ARG_REG;
+		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 {
+		pr_warn("usdt: unrecognized arg #%d spec '%s'\n", arg_num, arg_str);
+		return -EINVAL;
+	}
+
+	return len;
+}
+
 #else
 
 static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg, int *arg_sz)