tracing/osnoise: Fix crash in timerlat_dump_stack()

In the Linux kernel, the following vulnerability has been resolved:

tracing/osnoise: Fix crash in timerlat_dump_stack()

We have observed kernel panics when using timerlat with stack saving,

with the following dmesg output:

memcpy: detected buffer overflow: 88 byte write of buffer size 0

WARNING: CPU: 2 PID: 8153 at lib/string_helpers.c:1032 __fortify_report+0x55/0xa0

CPU: 2 UID: 0 PID: 8153 Comm: timerlatu/2 Kdump: loaded Not tainted 6.15.3-200.fc42.x86_64 #1 PREEMPT(lazy)

Call Trace:

<TASK>

? trace_buffer_lock_reserve+0x2a/0x60

__fortify_panic+0xd/0xf

__timerlat_dump_stack.cold+0xd/0xd

timerlat_dump_stack.part.0+0x47/0x80

timerlat_fd_read+0x36d/0x390

vfs_read+0xe2/0x390

? syscall_exit_to_user_mode+0x1d5/0x210

ksys_read+0x73/0xe0

do_syscall_64+0x7b/0x160

? exc_page_fault+0x7e/0x1a0

entry_SYSCALL_64_after_hwframe+0x76/0x7e

__timerlat_dump_stack() constructs the ftrace stack entry like this:

struct stack_entry *entry;

...

memcpy(&entry->caller, fstack->calls, size);

entry->size = fstack->nr_entries;

Since commit e7186af7fb26 ("tracing: Add back FORTIFY_SOURCE logic to

kernel_stack event structure"), struct stack_entry marks its caller

field with __counted_by(size). At the time of the memcpy, entry->size

contains garbage from the ringbuffer, which under some circumstances is

zero, triggering a kernel panic by buffer overflow.

Populate the size field before the memcpy so that the out-of-bounds

check knows the correct size. This is analogous to

__ftrace_trace_stack().