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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Mar 3, 2023 at 12:37 AM Puranjay Mohan <puranjay12@gmail.com> wrote:
>
> 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>
> ---

You don't mention that in the commit message, but how did you test
these changes?

> 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 - Resending as V1 shows up as Superseded in patchwork.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;
> +}
> +

let's use a more tabular approach, just like, say, riscv does?

> +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) {

if long function call is wrapped, argument on new line should be
aligned with the first argument on previous line. I'd suggest wrapping
right after format string, and start with &arg_sz aligned with arg_str

> +               /* 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) {

is the '#<num>' value always in decimal or it could be hex sometimes?

> +               /* 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;
> +       }

This part is repeated verbatim for each architecture, perhaps it's
better to do this post-processing and checking in parse_usdt_spec().
Would you mind adding another patch to your series that refactors
parse_usdt_arg() implementation to fill out struct usdt_arg_spec and
return arg_sz as out parameter. And then parse_usdt_spec() will check
arg_sz, set arg_signed and arg_bitshift parts?

> +
> +       return len;
> +}
> +
>  #else
>
>  static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg)
> --
> 2.39.1
>