[PATCH v2 bpf-next] libbpf: usdt arm arg parsing support

[Puranjay Mohan <puranjay12@gmail.com>]

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.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
---
Changes in V1[1] to V2
- Resending as V1 shows up as Superseded in patchwork.

[1] https://patchwork.kernel.org/project/netdevbpf/patch/20230220212741.13515-1-puranjay12@gmail.com/
---
 tools/lib/bpf/usdt.c | 82 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index 75b411fc2c77..ef097b882a4d 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -1505,6 +1505,88 @@ 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)
+{
+	int reg_num;
+
+	if (sscanf(reg_name, "r%d", &reg_num) == 1) {
+		if (reg_num >= 0 && reg_num <= 10)
+			return offsetof(struct pt_regs, uregs[reg_num]);
+	} else if (strcmp(reg_name, "fp") == 0) {
+		return offsetof(struct pt_regs, ARM_fp);
+	} else if (strcmp(reg_name, "ip") == 0) {
+		return offsetof(struct pt_regs, ARM_ip);
+	} else if (strcmp(reg_name, "sp") == 0) {
+		return offsetof(struct pt_regs, ARM_sp);
+	} else if (strcmp(reg_name, "lr") == 0) {
+		return offsetof(struct pt_regs, ARM_lr);
+	} else if (strcmp(reg_name, "pc") == 0) {
+		return offsetof(struct pt_regs, ARM_pc);
+	}
+	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)
+{
+	char reg_name[16];
+	int arg_sz, 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;
+	}
+
+	arg->arg_signed = arg_sz < 0;
+	if (arg_sz < 0)
+		arg_sz = -arg_sz;
+
+	switch (arg_sz) {
+	case 1: case 2: case 4: case 8:
+		arg->arg_bitshift = 64 - arg_sz * 8;
+		break;
+	default:
+		pr_warn("usdt: unsupported arg #%d (spec '%s') size: %d\n",
+			arg_num, arg_str, arg_sz);
+		return -EINVAL;
+	}
+
+	return len;
+}
+
 #else
 
 static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg)
-- 
2.39.1