[PATCH v2 bpf-next 0/1] libbpf: usdt aarch64 support

[PATCH v2 bpf-next 0/1] libbpf: usdt aarch64 support

[Alan Maguire]

USDT support [1] requires architecture-specific handling for translating
from argument strings describing each probe argument to appropriate values
that can be made available to the BPF program.  Determining value size,
whether it refers to a dereference (and if there is an offset from the
register value), a register value or a constant all have to be parsed
slightly differently for different architectures.  Details of format
handling for the aarch64 case are in patch 1.

Changes since v1:

- dropped patch refactoring all arch-specific parsing (Andrii)
- reworked sscanf()s to separate the two register dereference cases,
  made register parsing match [a-z0-9] in all cases (Andrii)

[1] https://lore.kernel.org/bpf/20220404234202.331384-1-andrii@kernel.org/

Alan Maguire (1):
  libbpf: usdt aarch64 arg parsing support

 tools/lib/bpf/usdt.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)

-- 
1.8.3.1

[PATCH v2 bpf-next] libbpf: usdt aarch64 arg parsing support

[Alan Maguire]

Parsing of USDT arguments is architecture-specific; on aarch64 it is
relatively easy since registers used are x[0-31], sp.  Format is
slightly different compared to x86_64; forms are

- "size @ [ reg[,offset] ]" for dereferences, for example
  "-8 @ [ sp, 76 ]" ; " -4 @ [ sp ]"
- "size @ reg" for register values; for example
  "-4@x0"
- "size @ value" for raw values; for example
  "-8@1"

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/lib/bpf/usdt.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)

diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index acf2d99..934c253 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -1324,6 +1324,82 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec
 	return len;
 }
 
+#elif defined(__aarch64__)
+
+static int calc_pt_regs_off(const char *reg_name)
+{
+	int reg_num;
+
+	if (sscanf(reg_name, "x%d", &reg_num) == 1) {
+		if (reg_num >= 0 && reg_num < 31)
+			return offsetof(struct user_pt_regs, regs[reg_num]);
+	} else if (strcmp(reg_name, "sp") == 0) {
+		return offsetof(struct user_pt_regs, sp);
+	}
+	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 = NULL;
+	int arg_sz, len, reg_off;
+	long off;
+
+	if (sscanf(arg_str, " %d @ \[ %m[a-z0-9], %ld ] %n", &arg_sz, &reg_name, &off, &len) == 3) {
+		/* Memory dereference case, e.g., -4@[sp, 96] */
+		arg->arg_type = USDT_ARG_REG_DEREF;
+		arg->val_off = off;
+		reg_off = calc_pt_regs_off(reg_name);
+		free(reg_name);
+		if (reg_off < 0)
+			return reg_off;
+		arg->reg_off = reg_off;
+	} else if (sscanf(arg_str, " %d @ \[ %m[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);
+		free(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 @ %m[a-z0-9] %n", &arg_sz, &reg_name, &len) == 2) {
+		/* Register read case, e.g., -8@x4 */
+		arg->arg_type = USDT_ARG_REG;
+		arg->val_off = 0;
+		reg_off = calc_pt_regs_off(reg_name);
+		free(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)
-- 
1.8.3.1

Re: [PATCH v2 bpf-next] libbpf: usdt aarch64 arg parsing support

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Mon, 11 Apr 2022 16:21:36 +0100 you wrote:
> Parsing of USDT arguments is architecture-specific; on aarch64 it is
> relatively easy since registers used are x[0-31], sp.  Format is
> slightly different compared to x86_64; forms are
> 
> - "size @ [ reg[,offset] ]" for dereferences, for example
>   "-8 @ [ sp, 76 ]" ; " -4 @ [ sp ]"
> - "size @ reg" for register values; for example
>   "-4@x0"
> - "size @ value" for raw values; for example
>   "-8@1"
> 
> [...]

Here is the summary with links:
  - [v2,bpf-next] libbpf: usdt aarch64 arg parsing support
    https://git.kernel.org/bpf/bpf-next/c/0f8619929c57

You are awesome, thank you!
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