bpf: Factor out stack_map build ID helpers

Factor out helpers from stack_map_get_build_id_offset() in
preparation for adding a sleepable build ID resolution path:
stack_map_build_id_set_ip(), stack_map_build_id_offset(), and
stack_map_build_id_set_valid().

While here, refactor stack_map_get_build_id_offset():
  * use continue-driven control flow in the main loop and remove
    build_id_valid label
  * update prev_vma and prev_build_id on the fall-back-to-IP branch so
    the cache reflects the actual VMA seen on the previous IP [1]
  * guard fetch_build_id() with vma_is_anonymous() [2] to skip parse
    attempts that would otherwise fail the ELF magic check

[1] https://lore.kernel.org/bpf/CAEf4Bzac9uWWqBvzH0iFzKvJcq3vxscZ3pKm0sUHmN-F-z9wVQ@mail.gmail.com/
[2] https://lore.kernel.org/bpf/226398c1ff3f2b686c0aeb010408d85fb15df13f9ff60a045bee31e79b9e41e9@mail.kernel.org/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/bpf/20260525223948.1920986-2-ihor.solodrai@linux.dev

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index da3d328f5c15a16767aa266ba7e73dcc41bd6713..e23be7d4450385405ec3189e2ff696704161d430 100644 (file)
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -152,6 +152,28 @@ static int fetch_build_id(struct vm_area_struct *vma, unsigned char *build_id, b
                         : build_id_parse_nofault(vma, build_id, NULL);
 }
 
+static inline void stack_map_build_id_set_ip(struct bpf_stack_build_id *id)
+{
+       id->status = BPF_STACK_BUILD_ID_IP;
+       memset(id->build_id, 0, BUILD_ID_SIZE_MAX);
+}
+
+static inline u64 stack_map_build_id_offset(unsigned long vm_pgoff,
+                                           unsigned long vm_start, u64 ip)
+{
+       return (vm_pgoff << PAGE_SHIFT) + ip - vm_start;
+}
+
+static inline void stack_map_build_id_set_valid(struct bpf_stack_build_id *id,
+                                               u64 offset,
+                                               const unsigned char *build_id)
+{
+       id->status = BPF_STACK_BUILD_ID_VALID;
+       id->offset = offset;
+       if (id->build_id != build_id)
+               memcpy(id->build_id, build_id, BUILD_ID_SIZE_MAX);
+}
+
 /*
  * Expects all id_offs[i].ip values to be set to correct initial IPs.
  * They will be subsequently:
@@ -165,44 +187,45 @@ static int fetch_build_id(struct vm_area_struct *vma, unsigned char *build_id, b
 static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
                                          u32 trace_nr, bool user, bool may_fault)
 {
-       int i;
        struct mmap_unlock_irq_work *work = NULL;
        bool irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
+       bool has_user_ctx = user && current && current->mm;
        struct vm_area_struct *vma, *prev_vma = NULL;
-       const char *prev_build_id;
+       const unsigned char *prev_build_id = NULL;
+       int i;
 
        /* If the irq_work is in use, fall back to report ips. Same
         * fallback is used for kernel stack (!user) on a stackmap with
         * build_id.
         */
-       if (!user || !current || !current->mm || irq_work_busy ||
-           !mmap_read_trylock(current->mm)) {
+       if (!has_user_ctx || irq_work_busy || !mmap_read_trylock(current->mm)) {
                /* cannot access current->mm, fall back to ips */
-               for (i = 0; i < trace_nr; i++) {
-                       id_offs[i].status = BPF_STACK_BUILD_ID_IP;
-                       memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
-               }
+               for (i = 0; i < trace_nr; i++)
+                       stack_map_build_id_set_ip(&id_offs[i]);
                return;
        }
 
        for (i = 0; i < trace_nr; i++) {
                u64 ip = READ_ONCE(id_offs[i].ip);
+               u64 offset;
 
-               if (range_in_vma(prev_vma, ip, ip)) {
+               if (prev_build_id && range_in_vma(prev_vma, ip, ip)) {
                        vma = prev_vma;
-                       memcpy(id_offs[i].build_id, prev_build_id, BUILD_ID_SIZE_MAX);
-                       goto build_id_valid;
+                       offset = stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip);
+                       stack_map_build_id_set_valid(&id_offs[i], offset, prev_build_id);
+                       continue;
                }
                vma = find_vma(current->mm, ip);
-               if (!vma || fetch_build_id(vma, id_offs[i].build_id, may_fault)) {
+               if (!vma || vma_is_anonymous(vma) ||
+                   fetch_build_id(vma, id_offs[i].build_id, may_fault)) {
                        /* per entry fall back to ips */
-                       id_offs[i].status = BPF_STACK_BUILD_ID_IP;
-                       memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
+                       stack_map_build_id_set_ip(&id_offs[i]);
+                       prev_vma = vma;
+                       prev_build_id = NULL;
                        continue;
                }
-build_id_valid:
-               id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ip - vma->vm_start;
-               id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
+               offset = stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip);
+               stack_map_build_id_set_valid(&id_offs[i], offset, id_offs[i].build_id);
                prev_vma = vma;
                prev_build_id = id_offs[i].build_id;
        }