[PATCH v5 8/9] tracing: Have deferred user space stacktrace show file offsets

Steven Rostedt
2025-04-24T19:24:18Z

From: Steven Rostedt <rostedt@goodmis.org>

Instead of showing the IP address of the user space stack trace, which is
where ever it was mapped by the kernel, show the offsets of where it would
be in the file.

Instead of:

trace-cmd-1066 [007] ..... 67.770256: <user stack unwind>
cookie=7000000000009
=> <00007fdbd0d421ca>
=> <00007fdbd0f3be27>
=> <00005635ece557e7>
=> <00005635ece559d3>
=> <00005635ece56523>
=> <00005635ece6479d>
=> <00005635ece64b01>
=> <00005635ece64bc0>
=> <00005635ece53b7e>
=> <00007fdbd0c6bca8>

Which is the addresses of the functions in the virtual address space of
the process. Have it record:

trace-cmd-1090 [003] ..... 180.779876: <user stack unwind>
cookie=3000000000009
=> <00000000001001ca>
=> <000000000000ae27>
=> <00000000000107e7>
=> <00000000000109d3>
=> <0000000000011523>
=> <000000000001f79d>
=> <000000000001fb01>
=> <000000000001fbc0>
=> <000000000000eb7e>
=> <0000000000029ca8>

Which is the offset from code where it was mapped at. To find this
address, the mmap_read_lock is taken and the vma is searched for the
addresses. Then what is recorded is simply:

(addr - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
kernel/trace/trace.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 71340207321e..f9eb0f7d649c 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3085,18 +3085,27 @@ static void trace_user_unwind_callback(struct unwind_work *unwind,
struct trace_buffer *buffer = tr->array_buffer.buffer;
struct userunwind_stack_entry *entry;
struct ring_buffer_event *event;
+ struct mm_struct *mm = current->mm;
unsigned int trace_ctx;
+ struct vm_area_struct *vma = NULL;
unsigned long *caller;
unsigned int offset;
int len;
int i;

+ /* This should never happen */
+ if (!mm)
+ return;
+
if (!(tr->trace_flags & TRACE_ITER_USERSTACKTRACE_DELAY))
return;

len = trace->nr * sizeof(unsigned long) + sizeof(*entry);

trace_ctx = tracing_gen_ctx();
+
+ guard(mmap_read_lock)(mm);
+
event = __trace_buffer_lock_reserve(buffer, TRACE_USER_UNWIND_STACK,
len, trace_ctx);
if (!event)
@@ -3113,7 +3122,16 @@ static void trace_user_unwind_callback(struct unwind_work *unwind,
caller = (void *)entry + offset;

for (i = 0; i < trace->nr; i++) {
- caller[i] = trace->entries[i];
+ unsigned long addr = trace->entries[i];
+
+ if (!vma || addr < vma->vm_start || addr >= vma->vm_end)
+ vma = vma_lookup(mm, addr);
+
+ if (!vma) {
+ caller[i] = addr;
+ continue;
+ }
+ caller[i] = (addr - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);
}

__buffer_unlock_commit(buffer, event);
--
2.47.2