[PATCH v6 bpf-next 05/10] lib/buildid: rename build_id_parse() into build_id_parse_nofault()

[Andrii Nakryiko <andrii@kernel.org>]

Make it clear that build_id_parse() assumes that it can take no page
fault by renaming it and current few users to build_id_parse_nofault().

Also add build_id_parse() stub which for now falls back to non-sleepable
implementation, but will be changed in subsequent patches to take
advantage of sleepable context. PROCMAP_QUERY ioctl() on
/proc/<pid>/maps file is using build_id_parse() and will automatically
take advantage of more reliable sleepable context implementation.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 include/linux/buildid.h |  4 ++--
 kernel/bpf/stackmap.c   |  2 +-
 kernel/events/core.c    |  2 +-
 lib/buildid.c           | 25 ++++++++++++++++++++++---
 4 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/include/linux/buildid.h b/include/linux/buildid.h
index 20aa3c2d89f7..014a88c41073 100644
--- a/include/linux/buildid.h
+++ b/include/linux/buildid.h
@@ -7,8 +7,8 @@
 #define BUILD_ID_SIZE_MAX 20
 
 struct vm_area_struct;
-int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
-		   __u32 *size);
+int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size);
+int build_id_parse_nofault(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size);
 int build_id_parse_buf(const void *buf, unsigned char *build_id, u32 buf_size);
 
 #if IS_ENABLED(CONFIG_STACKTRACE_BUILD_ID) || IS_ENABLED(CONFIG_VMCORE_INFO)
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index c99f8e5234ac..770ae8e88016 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -156,7 +156,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 			goto build_id_valid;
 		}
 		vma = find_vma(current->mm, ips[i]);
-		if (!vma || build_id_parse(vma, id_offs[i].build_id, NULL)) {
+		if (!vma || build_id_parse_nofault(vma, id_offs[i].build_id, NULL)) {
 			/* per entry fall back to ips */
 			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
 			id_offs[i].ip = ips[i];
diff --git a/kernel/events/core.c b/kernel/events/core.c
index aa3450bdc227..c263a8b0ce54 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8851,7 +8851,7 @@ static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
 	mmap_event->event_id.header.size = sizeof(mmap_event->event_id) + size;
 
 	if (atomic_read(&nr_build_id_events))
-		build_id_parse(vma, mmap_event->build_id, &mmap_event->build_id_size);
+		build_id_parse_nofault(vma, mmap_event->build_id, &mmap_event->build_id_size);
 
 	perf_iterate_sb(perf_event_mmap_output,
 		       mmap_event,
diff --git a/lib/buildid.c b/lib/buildid.c
index e8fc4aeb01f2..c1cbd34f3685 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -293,10 +293,12 @@ static int get_build_id_64(struct freader *r, unsigned char *build_id, __u32 *si
  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
  * @size:     returns actual build id size in case of success
  *
- * Return: 0 on success, -EINVAL otherwise
+ * Assumes no page fault can be taken, so if relevant portions of ELF file are
+ * not already paged in, fetching of build ID fails.
+ *
+ * Return: 0 on success; negative error, otherwise
  */
-int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
-		   __u32 *size)
+int build_id_parse_nofault(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 {
 	const Elf32_Ehdr *ehdr;
 	struct freader r;
@@ -335,6 +337,23 @@ int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
 	return ret;
 }
 
+/*
+ * Parse build ID of ELF file mapped to VMA
+ * @vma:      vma object
+ * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
+ * @size:     returns actual build id size in case of success
+ *
+ * Assumes faultable context and can cause page faults to bring in file data
+ * into page cache.
+ *
+ * Return: 0 on success; negative error, otherwise
+ */
+int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
+{
+	/* fallback to non-faultable version for now */
+	return build_id_parse_nofault(vma, build_id, size);
+}
+
 /**
  * build_id_parse_buf - Get build ID from a buffer
  * @buf:      ELF note section(s) to parse
-- 
2.43.5