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tracing/probes: Add $current variable support

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Since we can use the BTF to cast value to a structure pointer type,
it is useful to introduce "$current" special variable support to
fetcharg.

User can define a fetcharg to access current task_struct properties
using BTF info. e.g.

  $current->cpus_ptr

Link: https://lore.kernel.org/all/178271366709.1176915.15320906169981578568.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
This commit is contained in:
Masami Hiramatsu (Google)
2026-07-02 21:12:41 +09:00
parent f218345240
commit 738b78ee4b
6 changed files with 44 additions and 3 deletions
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4
kernel/trace/trace.c
View File

@@ -4320,13 +4320,13 @@ static const char readme_msg[] =
"\t args: <name>=fetcharg[:type]\n"
"\t fetcharg: (%<register>|$<efield>), @<address>, @<symbol>[+|-<offset>],\n"
#ifdef CONFIG_HAVE_FUNCTION_ARG_ACCESS_API
"\t $stack<index>, $stack, $retval, $comm, $arg<N>,\n"
"\t $stack<index>, $stack, $retval, $comm, $arg<N>, $current\n"
#ifdef CONFIG_PROBE_EVENTS_BTF_ARGS
"\t [(structname[,field])]<argname>[->field[->field|.field...]],\n"
"\t [(structname[,field])](fetcharg)->field[->field|.field...],\n"
#endif
#else
"\t $stack<index>, $stack, $retval, $comm,\n"
"\t $stack<index>, $stack, $retval, $comm, $current\n"
#endif
"\t +|-[u]<offset>(<fetcharg>), \\imm-value, \\\"imm-string\"\n"
"\t kernel return probes support: $retval, $arg<N>, $comm\n"

37
kernel/trace/trace_probe.c
View File

@@ -692,7 +692,9 @@ static int parse_btf_arg(char *varname,
int i, is_ptr, ret;
u32 tid;
if (!ctx->funcname && !(ctx->flags & TPARG_FL_TEVENT))
/* Note: field is not separated at this point, so check prefix. */
if (!str_has_prefix(varname, "$current") &&
!ctx->funcname && !(ctx->flags & TPARG_FL_TEVENT))
return -EINVAL;
is_ptr = split_next_field(varname, &field, ctx);
@@ -705,6 +707,20 @@ static int parse_btf_arg(char *varname,
return -EOPNOTSUPP;
}
if (!strcmp(varname, "$current")) {
code->op = FETCH_OP_CURRENT;
/* If no typecast is specified for $current, use task_struct by default */
ret = bpf_find_btf_id("task_struct", BTF_KIND_STRUCT, &ctx->struct_btf);
if (ret < 0) {
trace_probe_log_err(ctx->offset, NO_BTF_ENTRY);
return -ENOENT;
}
tid = (u32)ret;
type = ctx->last_struct =
btf_type_skip_modifiers(ctx->struct_btf, tid, NULL);
goto found_type;
}
if (ctx->flags & TPARG_FL_RETURN && !strcmp(varname, "$retval")) {
code->op = FETCH_OP_RETVAL;
/* Check whether the function return type is not void, even with typecast. */
@@ -761,6 +777,7 @@ static int parse_btf_arg(char *varname,
found:
type = btf_type_skip_modifiers(ctx->btf, tid, NULL);
found_type:
if (!type) {
trace_probe_log_err(ctx->offset, BAD_BTF_TID);
return -EINVAL;
@@ -1270,6 +1287,24 @@ static int parse_probe_vars(char *orig_arg, const struct fetch_type *t,
return 0;
}
/* $current returns the address of the current task_struct. */
if (str_has_prefix(arg, "current")) {
/* $current is only supported by kernel probe. */
if (!(ctx->flags & TPARG_FL_KERNEL)) {
err = TP_ERR_BAD_VAR;
goto inval;
}
arg += strlen("current");
if (*arg == '-' && IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS))
return parse_btf_arg(orig_arg, pcode, end, ctx);
if (*arg != '\0')
goto inval;
code->op = FETCH_OP_CURRENT;
return 0;
}
#ifdef CONFIG_HAVE_FUNCTION_ARG_ACCESS_API
len = str_has_prefix(arg, "arg");
if (len) {

1
kernel/trace/trace_probe.h
View File

@@ -92,6 +92,7 @@ typedef int (*print_type_func_t)(struct trace_seq *, void *, void *);
FETCH_OP(RETVAL, none), /* Return value */ \
FETCH_OP(IMM, imm), /* Immediate: .immediate */ \
FETCH_OP(COMM, none), /* Current comm */ \
FETCH_OP(CURRENT, none), /* Current task_struct address */\
FETCH_OP(ARG, param), /* Argument: .param = index */ \
FETCH_OP(FOFFS, imm), /* File offset: .immediate */ \
FETCH_OP(IMMSTR, string), /* Allocated string: .data */ \

3
kernel/trace/trace_probe_tmpl.h
View File

@@ -112,6 +112,9 @@ process_common_fetch_insn(struct fetch_insn *code, unsigned long *val)
case FETCH_OP_IMMSTR:
*val = (unsigned long)code->data;
break;
case FETCH_OP_CURRENT:
*val = (unsigned long)current;
break;
default:
return -EILSEQ;
}