[PATCH 5/9 v3.1] tracing/probes: Support BTF field access from $retval

Masami Hiramatsu (Google)
2023-07-27T01:59:17Z

Support BTF argument on '$retval' for function return events including
kretprobe and fprobe for accessing the return value.
This also allows user to access its fields if the return value is a
pointer of a data structure.

E.g.
# echo 'f getname_flags%return +0($retval->name):string' \
> dynamic_events
# echo 1 > events/fprobes/getname_flags__exit/enable
# ls > /dev/null
# head -n 40 trace | tail
ls-87 [000] ...1. 8067.616101: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./function_profile_enabled"
ls-87 [000] ...1. 8067.616108: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./trace_stat"
ls-87 [000] ...1. 8067.616115: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_graph_notrace"
ls-87 [000] ...1. 8067.616122: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_graph_function"
ls-87 [000] ...1. 8067.616129: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_ftrace_notrace"
ls-87 [000] ...1. 8067.616135: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_ftrace_filter"
ls-87 [000] ...1. 8067.616143: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./touched_functions"
ls-87 [000] ...1. 8067.616237: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./enabled_functions"
ls-87 [000] ...1. 8067.616245: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./available_filter_functions"
ls-87 [000] ...1. 8067.616253: getname_flags__exit: (vfs_fstatat+0x3c/0x70 <- getname_flags) arg1="./set_ftrace_notrace_pid"

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v2:
- Use '$retval' instead of 'retval' because it is confusing.
Changes in v3:
- Introduce query_btf_context() to cache the btf related data (function
prototype) for using common field analyzing code with function
parameters.
Changes in v3.1
- Return int error code from query_btf_context() if !CONFIG_PROBE_EVENTS_BTF_ARGS
---
kernel/trace/trace_probe.c | 182 +++++++++++++++++++-------------------------
kernel/trace/trace_probe.h | 1
2 files changed, 81 insertions(+), 102 deletions(-)

diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index f6b855de4256..6fef0c08be63 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -363,38 +363,46 @@ static const char *fetch_type_from_btf_type(struct btf *btf,
return NULL;
}

-static const struct btf_param *find_btf_func_param(const char *funcname, s32 *nr,
- struct btf **btf_p, bool tracepoint)
+static int query_btf_context(struct traceprobe_parse_context *ctx)
{
const struct btf_param *param;
- const struct btf_type *t;
+ const struct btf_type *type;
struct btf *btf;
+ s32 nr;

- if (!funcname || !nr)
- return ERR_PTR(-EINVAL);
+ if (ctx->btf)
+ return 0;

- t = btf_find_func_proto(funcname, &btf);
- if (!t)
- return (const struct btf_param *)t;
+ if (!ctx->funcname)
+ return -EINVAL;

- param = btf_get_func_param(t, nr);
- if (IS_ERR_OR_NULL(param))
- goto err;
+ type = btf_find_func_proto(ctx->funcname, &btf);
+ if (!type)
+ return -ENOENT;

- /* Hide the first 'data' argument of tracepoint */
- if (tracepoint) {
- (*nr)--;
- param++;
+ ctx->btf = btf;
+ ctx->proto = type;
+
+ /* ctx->params is optional, since func(void) will not have params. */
+ nr = 0;
+ param = btf_get_func_param(type, &nr);
+ if (!IS_ERR_OR_NULL(param)) {
+ /* Hide the first 'data' argument of tracepoint */
+ if (ctx->flags & TPARG_FL_TPOINT) {
+ nr--;
+ param++;
+ }
}

- if (*nr > 0) {
- *btf_p = btf;
- return param;
+ if (nr > 0) {
+ ctx->nr_params = nr;
+ ctx->params = param;
+ } else {
+ ctx->nr_params = 0;
+ ctx->params = NULL;
}

-err:
- btf_put(btf);
- return NULL;
+ return 0;
}

static void clear_btf_context(struct traceprobe_parse_context *ctx)
@@ -402,6 +410,7 @@ static void clear_btf_context(struct traceprobe_parse_context *ctx)
if (ctx->btf) {
btf_put(ctx->btf);
ctx->btf = NULL;
+ ctx->proto = NULL;
ctx->params = NULL;
ctx->nr_params = 0;
}
@@ -517,7 +526,7 @@ static int parse_btf_arg(char *varname,
const struct btf_param *params;
const struct btf_type *type;
char *field = NULL;
- int i, is_ptr;
+ int i, is_ptr, ret;
u32 tid;

if (WARN_ON_ONCE(!ctx->funcname))
@@ -533,17 +542,32 @@ static int parse_btf_arg(char *varname,
return -EOPNOTSUPP;
}

- if (!ctx->params) {
- params = find_btf_func_param(ctx->funcname,
- &ctx->nr_params, &ctx->btf,
- ctx->flags & TPARG_FL_TPOINT);
- if (IS_ERR_OR_NULL(params)) {
+ if (ctx->flags & TPARG_FL_RETURN) {
+ if (strcmp(varname, "$retval") != 0) {
+ trace_probe_log_err(ctx->offset, NO_BTFARG);
+ return -ENOENT;
+ }
+ /* Check whether the function return type is not void */
+ if (query_btf_context(ctx) == 0) {
+ if (ctx->proto->type == 0) {
+ trace_probe_log_err(ctx->offset, NO_RETVAL);
+ return -ENOENT;
+ }
+ tid = ctx->proto->type;
+ } else
+ tid = 0;
+ code->op = FETCH_OP_RETVAL;
+ goto found;
+ }
+
+ if (!ctx->btf) {
+ ret = query_btf_context(ctx);
+ if (ret < 0 || ctx->nr_params == 0) {
trace_probe_log_err(ctx->offset, NO_BTF_ENTRY);
return PTR_ERR(params);
}
- ctx->params = params;
- } else
- params = ctx->params;
+ }
+ params = ctx->params;

for (i = 0; i < ctx->nr_params; i++) {
const char *name = btf_name_by_offset(ctx->btf, params[i].name_off);
@@ -554,7 +578,6 @@ static int parse_btf_arg(char *varname,
code->param = i + 1;
else
code->param = i;
-
tid = params[i].type;
goto found;
}
@@ -579,7 +602,7 @@ static int parse_btf_arg(char *varname,
return 0;
}

-static const struct fetch_type *parse_btf_arg_type(
+static const struct fetch_type *find_fetch_type_from_btf_type(
struct traceprobe_parse_context *ctx)
{
struct btf *btf = ctx->btf;
@@ -591,27 +614,6 @@ static const struct fetch_type *parse_btf_arg_type(
return find_fetch_type(typestr, ctx->flags);
}

-static const struct fetch_type *parse_btf_retval_type(
- struct traceprobe_parse_context *ctx)
-{
- const char *typestr = NULL;
- const struct btf_type *type;
- struct btf *btf;
-
- if (ctx->funcname) {
- /* Do not use ctx->btf, because it must be used with ctx->param */
- type = btf_find_func_proto(ctx->funcname, &btf);
- if (type) {
- type = btf_type_skip_modifiers(btf, type->type, NULL);
- if (!IS_ERR_OR_NULL(type))
- typestr = fetch_type_from_btf_type(btf, type, ctx);
- btf_put(btf);
- }
- }
-
- return find_fetch_type(typestr, ctx->flags);
-}
-
static int parse_btf_bitfield(struct fetch_insn **pcode,
struct traceprobe_parse_context *ctx)
{
@@ -634,30 +636,15 @@ static int parse_btf_bitfield(struct fetch_insn **pcode,
return 0;
}

-static bool is_btf_retval_void(const char *funcname)
-{
- const struct btf_type *t;
- struct btf *btf;
- bool ret;
-
- t = btf_find_func_proto(funcname, &btf);
- if (!t)
- return false;
-
- ret = (t->type == 0);
- btf_put(btf);
- return ret;
-}
#else
static void clear_btf_context(struct traceprobe_parse_context *ctx)
{
ctx->btf = NULL;
}

-static const struct btf_param *find_btf_func_param(const char *funcname, s32 *nr,
- struct btf **btf_p, bool tracepoint)
+static int query_btf_context(struct traceprobe_parse_context *ctx)
{
- return ERR_PTR(-EOPNOTSUPP);
+ return -EOPNOTSUPP;
}

static int parse_btf_arg(char *varname,
@@ -675,24 +662,23 @@ static int parse_btf_bitfield(struct fetch_insn **pcode,
return -EOPNOTSUPP;
}

-#define parse_btf_arg_type(ctx) \
+#define find_fetch_type_from_btf_type(ctx) \
find_fetch_type(NULL, ctx->flags)

-#define parse_btf_retval_type(ctx) \
- find_fetch_type(NULL, ctx->flags)
-
-#define is_btf_retval_void(funcname) (false)
-
#endif

#define PARAM_MAX_STACK (THREAD_SIZE / sizeof(unsigned long))

-static int parse_probe_vars(char *arg, const struct fetch_type *t,
- struct fetch_insn *code,
+/* Parse $vars. @orig_arg points '$', which syncs to @ctx->offset */
+static int parse_probe_vars(char *orig_arg, const struct fetch_type *t,
+ struct fetch_insn **pcode,
+ struct fetch_insn *end,
struct traceprobe_parse_context *ctx)
{
- unsigned long param;
+ struct fetch_insn *code = *pcode;
int err = TP_ERR_BAD_VAR;
+ char *arg = orig_arg + 1;
+ unsigned long param;
int ret = 0;
int len;

@@ -711,18 +697,17 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
goto inval;
}

- if (strcmp(arg, "retval") == 0) {
- if (ctx->flags & TPARG_FL_RETURN) {
- if ((ctx->flags & TPARG_FL_KERNEL) &&
- is_btf_retval_void(ctx->funcname)) {
- err = TP_ERR_NO_RETVAL;
- goto inval;
- }
+ if (str_has_prefix(arg, "retval")) {
+ if (!(ctx->flags & TPARG_FL_RETURN)) {
+ err = TP_ERR_RETVAL_ON_PROBE;
+ goto inval;
+ }
+ if (!(ctx->flags & TPARG_FL_KERNEL) ||
+ !IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS)) {
code->op = FETCH_OP_RETVAL;
return 0;
}
- err = TP_ERR_RETVAL_ON_PROBE;
- goto inval;
+ return parse_btf_arg(orig_arg, pcode, end, ctx);
}

len = str_has_prefix(arg, "stack");
@@ -824,7 +809,7 @@ parse_probe_arg(char *arg, const struct fetch_type *type,

switch (arg[0]) {
case '$':
- ret = parse_probe_vars(arg + 1, type, code, ctx);
+ ret = parse_probe_vars(arg, type, pcode, end, ctx);
break;

case '%': /* named register */
@@ -1121,12 +1106,9 @@ static int traceprobe_parse_probe_arg_body(const char *argv, ssize_t *size,
goto fail;

/* Update storing type if BTF is available */
- if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) && !t) {
- if (ctx->last_type)
- parg->type = parse_btf_arg_type(ctx);
- else if (ctx->flags & TPARG_FL_RETURN)
- parg->type = parse_btf_retval_type(ctx);
- }
+ if (IS_ENABLED(CONFIG_PROBE_EVENTS_BTF_ARGS) &&
+ !t && ctx->last_type)
+ parg->type = find_fetch_type_from_btf_type(ctx);

ret = -EINVAL;
/* Store operation */
@@ -1415,7 +1397,6 @@ const char **traceprobe_expand_meta_args(int argc, const char *argv[],
const struct btf_param *params = NULL;
int i, j, n, used, ret, args_idx = -1;
const char **new_argv = NULL;
- int nr_params;

ret = argv_has_var_arg(argc, argv, &args_idx, ctx);
if (ret < 0)
@@ -1426,9 +1407,8 @@ const char **traceprobe_expand_meta_args(int argc, const char *argv[],
return NULL;
}

- params = find_btf_func_param(ctx->funcname, &nr_params, &ctx->btf,
- ctx->flags & TPARG_FL_TPOINT);
- if (IS_ERR_OR_NULL(params)) {
+ ret = query_btf_context(ctx);
+ if (ret < 0 || ctx->nr_params == 0) {
if (args_idx != -1) {
/* $arg* requires BTF info */
trace_probe_log_err(0, NOSUP_BTFARG);
@@ -1437,8 +1417,6 @@ const char **traceprobe_expand_meta_args(int argc, const char *argv[],
*new_argc = argc;
return NULL;
}
- ctx->params = params;
- ctx->nr_params = nr_params;

if (args_idx >= 0)
*new_argc = argc + ctx->nr_params - 1;
@@ -1453,7 +1431,7 @@ const char **traceprobe_expand_meta_args(int argc, const char *argv[],
for (i = 0, j = 0; i < argc; i++) {
trace_probe_log_set_index(i + 2);
if (i == args_idx) {
- for (n = 0; n < nr_params; n++) {
+ for (n = 0; n < ctx->nr_params; n++) {
ret = sprint_nth_btf_arg(n, "", buf + used,
bufsize - used, ctx);
if (ret < 0)
diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
index 6111f1ffca6c..9184c84833f8 100644
--- a/kernel/trace/trace_probe.h
+++ b/kernel/trace/trace_probe.h
@@ -385,6 +385,7 @@ struct traceprobe_parse_context {
struct trace_event_call *event;
/* BTF related parameters */
const char *funcname; /* Function name in BTF */
+ const struct btf_type *proto; /* Prototype of the function */
const struct btf_param *params; /* Parameter of the function */
s32 nr_params; /* The number of the parameters */
struct btf *btf; /* The BTF to be used */