From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Add btf_find_struct_member() API to search a member of a given data structure
or union from the member's name.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
---
 Changes in v3:
  - Remove simple input check.
  - Fix unneeded IS_ERR_OR_NULL() check for btf_type_by_id().
  - Move the code next to btf_get_func_param().
  - Use for_each_member() macro instead of for-loop.
  - Use btf_type_skip_modifiers() instead of btf_type_by_id().
 Changes in v4:
  - Use a stack for searching in anonymous members instead of nested call.
 Changes in v5:
  - Move the generic functions into a new file, trace_btf.c, for avoiding
    tree-merge conflict.
  - Return anon_offset for calculating correct offset from root struct.
---
 kernel/trace/trace_btf.c |   57 ++++++++++++++++++++++++++++++++++++++++++++++
 kernel/trace/trace_btf.h |    4 +++
 2 files changed, 61 insertions(+)

diff --git a/kernel/trace/trace_btf.c b/kernel/trace/trace_btf.c
index 82cbbeeda071..109a238437cf 100644
--- a/kernel/trace/trace_btf.c
+++ b/kernel/trace/trace_btf.c
@@ -50,3 +50,60 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
 		return NULL;
 }
 
+#define BTF_ANON_STACK_MAX	16
+
+/*
+ * Find a member of data structure/union by name and return it.
+ * Return NULL if not found, or -EINVAL if parameter is invalid.
+ * If the member is an member of anonymous union/structure, the offset
+ * of that anonymous union/structure is stored into @anon_offset. Caller
+ * can calculate the correct offset from the root data structure by
+ * adding anon_offset to the member's offset.
+ */
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+						const struct btf_type *type,
+						const char *member_name,
+						u32 *anon_offset)
+{
+	struct {
+		u32 tid;
+		u32 offset;
+	} anon_stack[BTF_ANON_STACK_MAX];
+	const struct btf_member *member;
+	u32 tid, cur_offset = 0;
+	const char *name;
+	int i, top = 0;
+
+retry:
+	if (!btf_type_is_struct(type))
+		return ERR_PTR(-EINVAL);
+
+	for_each_member(i, type, member) {
+		if (!member->name_off) {
+			/* Anonymous union/struct: push it for later use */
+			type = btf_type_skip_modifiers(btf, member->type, &tid);
+			if (type && top < BTF_ANON_STACK_MAX) {
+				anon_stack[top].tid = tid;
+				anon_stack[top++].offset =
+					cur_offset + member->offset;
+			}
+		} else {
+			name = btf_name_by_offset(btf, member->name_off);
+			if (name && !strcmp(member_name, name)) {
+				if (anon_offset)
+					*anon_offset = cur_offset;
+				return member;
+			}
+		}
+	}
+	if (top > 0) {
+		/* Pop from the anonymous stack and retry */
+		tid = anon_stack[--top].tid;
+		cur_offset = anon_stack[top].offset;
+		type = btf_type_by_id(btf, tid);
+		goto retry;
+	}
+
+	return NULL;
+}
+
diff --git a/kernel/trace/trace_btf.h b/kernel/trace/trace_btf.h
index 2ef1b2367e1e..fb2919a74225 100644
--- a/kernel/trace/trace_btf.h
+++ b/kernel/trace/trace_btf.h
@@ -5,3 +5,7 @@ const struct btf_type *btf_find_func_proto(const char *func_name,
 					   struct btf **btf_p);
 const struct btf_param *btf_get_func_param(const struct btf_type *func_proto,
 					   s32 *nr);
+const struct btf_member *btf_find_struct_member(struct btf *btf,
+						const struct btf_type *type,
+						const char *member_name,
+						u32 *anon_offset);

On Mon,  7 Aug 2023 11:54:58 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> --- a/kernel/trace/trace_btf.c
> +++ b/kernel/trace/trace_btf.c
> @@ -50,3 +50,60 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
>  		return NULL;
>  }
>  
> +#define BTF_ANON_STACK_MAX	16
> +
> +/*
> + * Find a member of data structure/union by name and return it.
> + * Return NULL if not found, or -EINVAL if parameter is invalid.
> + * If the member is an member of anonymous union/structure, the offset
> + * of that anonymous union/structure is stored into @anon_offset. Caller
> + * can calculate the correct offset from the root data structure by
> + * adding anon_offset to the member's offset.
> + */
> +const struct btf_member *btf_find_struct_member(struct btf *btf,
> +						const struct btf_type *type,
> +						const char *member_name,
> +						u32 *anon_offset)
> +{
> +	struct {
> +		u32 tid;
> +		u32 offset;
> +	} anon_stack[BTF_ANON_STACK_MAX];

Where is this called as the above is 128 bytes, which is a bit large
for the stack. It may not be bad if it's not that generic of a
function. But if the stack is getting tight, this could still be an
issue.

-- Steve


> +	const struct btf_member *member;
> +	u32 tid, cur_offset = 0;
> +	const char *name;
> +	int i, top = 0;
> +
> +retry:
> +	if (!btf_type_is_struct(type))
> +		return ERR_PTR(-EINVAL);
> +
> +	for_each_member(i, type, member) {
> +		if (!member->name_off) {
> +			/* Anonymous union/struct: push it for later use */
> +			type = btf_type_skip_modifiers(btf, member->type, &tid);
> +			if (type && top < BTF_ANON_STACK_MAX) {
> +				anon_stack[top].tid = tid;
> +				anon_stack[top++].offset =
> +					cur_offset + member->offset;
> +			}
> +		} else {
> +			name = btf_name_by_offset(btf, member->name_off);
> +			if (name && !strcmp(member_name, name)) {
> +				if (anon_offset)
> +					*anon_offset = cur_offset;
> +				return member;
> +			}
> +		}
> +	}
> +	if (top > 0) {
> +		/* Pop from the anonymous stack and retry */
> +		tid = anon_stack[--top].tid;
> +		cur_offset = anon_stack[top].offset;
> +		type = btf_type_by_id(btf, tid);
> +		goto retry;
> +	}
> +
> +	return NULL;
> +}
> +

On Tue, 22 Aug 2023 09:37:20 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon,  7 Aug 2023 11:54:58 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> 
> > --- a/kernel/trace/trace_btf.c
> > +++ b/kernel/trace/trace_btf.c
> > @@ -50,3 +50,60 @@ const struct btf_param *btf_get_func_param(const struct btf_type *func_proto, s3
> >  		return NULL;
> >  }
> >  
> > +#define BTF_ANON_STACK_MAX	16
> > +
> > +/*
> > + * Find a member of data structure/union by name and return it.
> > + * Return NULL if not found, or -EINVAL if parameter is invalid.
> > + * If the member is an member of anonymous union/structure, the offset
> > + * of that anonymous union/structure is stored into @anon_offset. Caller
> > + * can calculate the correct offset from the root data structure by
> > + * adding anon_offset to the member's offset.
> > + */
> > +const struct btf_member *btf_find_struct_member(struct btf *btf,
> > +						const struct btf_type *type,
> > +						const char *member_name,
> > +						u32 *anon_offset)
> > +{
> > +	struct {
> > +		u32 tid;
> > +		u32 offset;
> > +	} anon_stack[BTF_ANON_STACK_MAX];
> 
> Where is this called as the above is 128 bytes, which is a bit large
> for the stack. It may not be bad if it's not that generic of a
> function. But if the stack is getting tight, this could still be an
> issue.

OK, let me allocate an array then.

Thank you,

> 
> -- Steve
> 
> 
> > +	const struct btf_member *member;
> > +	u32 tid, cur_offset = 0;
> > +	const char *name;
> > +	int i, top = 0;
> > +
> > +retry:
> > +	if (!btf_type_is_struct(type))
> > +		return ERR_PTR(-EINVAL);
> > +
> > +	for_each_member(i, type, member) {
> > +		if (!member->name_off) {
> > +			/* Anonymous union/struct: push it for later use */
> > +			type = btf_type_skip_modifiers(btf, member->type, &tid);
> > +			if (type && top < BTF_ANON_STACK_MAX) {
> > +				anon_stack[top].tid = tid;
> > +				anon_stack[top++].offset =
> > +					cur_offset + member->offset;
> > +			}
> > +		} else {
> > +			name = btf_name_by_offset(btf, member->name_off);
> > +			if (name && !strcmp(member_name, name)) {
> > +				if (anon_offset)
> > +					*anon_offset = cur_offset;
> > +				return member;
> > +			}
> > +		}
> > +	}
> > +	if (top > 0) {
> > +		/* Pop from the anonymous stack and retry */
> > +		tid = anon_stack[--top].tid;
> > +		cur_offset = anon_stack[top].offset;
> > +		type = btf_type_by_id(btf, tid);
> > +		goto retry;
> > +	}
> > +
> > +	return NULL;
> > +}
> > +


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>