Re: [RFC PATCH v1 00/25] unwind_user: Implement .eh_frame handling

[Steven Rostedt <rostedt@kernel.org>]

On Tue, 18 Aug 2026 16:49:29 +0200
Jens Remus <jremus@linux.ibm.com> wrote:

> This series adds support for parsing DWARF Call Frame Information (CFI)
> from the .eh_frame_hdr and .eh_frame sections of user space ELF files.

Very nice! I don't have time this week to look at it, but I just
applied the series and tried it out with:

  perf record --call-graph fp,defer trace-cmd report ~/trace.dat

And did a: perf -D script

And found this:

264990534603 0x19370 [0x78]: PERF_RECORD_CALLCHAIN_DEFERRED(IP, 0x2): 1165/1165: 0x105700000005
... FP chain: nr:10
.....  0: 00007f5feafd362a
.....  1: 00007f5feb249781
.....  2: 00007f5feb2498de
.....  3: 000055935526fbdc
.....  4: 00005593552649e6
.....  5: 000055935523c2d0
.....  6: 000055935523d24c
.....  7: 0000559355227bfe
.....  8: 00007f5feaef9f75
.....  9: 00007ffdaf879d38
 ... thread: trace-cmd:1165
 ...... dso: /proc/kcore
trace-cmd    1165   264.990473:     395990 cpu/cycles/P: 
        ffffffff8477d8d3 check_preemption_disabled+0x13 ([kernel.kallsyms])
        ffffffff819aa495 rcu_is_watching+0x15 ([kernel.kallsyms])
        ffffffff816dbcff unwind_next_frame+0x45f ([kernel.kallsyms])
        ffffffff81642e91 arch_stack_walk+0xa1 ([kernel.kallsyms])
        ffffffff81a2e633 stack_trace_save+0x93 ([kernel.kallsyms])
        ffffffff82251210 kasan_save_stack+0x30 ([kernel.kallsyms])
        ffffffff822541b0 kasan_record_aux_stack+0xb0 ([kernel.kallsyms])
        ffffffff819c1e2a __call_rcu_common+0xca ([kernel.kallsyms])
        ffffffff821be05f kmem_cache_free+0x2ef ([kernel.kallsyms])
        ffffffff8233e4a7 vfs_fstatat+0x57 ([kernel.kallsyms])
        ffffffff8233e573 __do_sys_newfstatat+0x83 ([kernel.kallsyms])
        ffffffff84772a8e do_syscall_64+0x7e ([kernel.kallsyms])
        ffffffff8100012f entry_SYSCALL_64_after_hwframe+0x76 ([kernel.kallsyms])
            7f5feafd362a __GI___fstatat64+0xa (/usr/lib/x86_64-linux-gnu/libc.so.6)
            7f5feb249781 tep_load_plugins_hook+0xd1 (/usr/local/lib64/libtraceevent.so.1.9.0)
            7f5feb2498de tep_load_plugins+0x35 (/usr/local/lib64/libtraceevent.so.1.9.0)
            55935526fbdc tcmd_load_plugins+0x85 (/usr/local/bin/trace-cmd)
            5593552649e6 tracecmd_alloc_fd+0x2da (/usr/local/bin/trace-cmd)
            55935523c2d0 read_trace_header+0x62 (/usr/local/bin/trace-cmd)
            55935523d24c trace_report+0x869 (/usr/local/bin/trace-cmd)
            559355227bfe main+0x90 (/usr/local/bin/trace-cmd)
            7f5feaef9f75 __libc_start_call_main+0x75 (/usr/lib/x86_64-linux-gnu/libc.so.6)
            7ffdaf879d38 [unknown] ([unknown])

I injected trace_printk() into the code to make sure it was using the
eh_frame unwinding:

diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index 85fc82252af1..585f022bcabe 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -200,6 +200,7 @@ static int unwind_user_next(struct unwind_user_state *state)
 		case UNWIND_USER_TYPE_EH_FRAME:
 			switch (unwind_user_next_eh_frame(state)) {
 			case 0:
+				trace_printk("USE EH_FRAME\n");
 				return 0;
 			case -ENOENT:
 				continue;	/* Try next method. */
@@ -208,8 +209,9 @@ static int unwind_user_next(struct unwind_user_state *state)
 			}
 			break;
 		case UNWIND_USER_TYPE_FP:
-			if (!unwind_user_next_fp(state))
-				return 0;
+			if (!unwind_user_next_fp(state)) {
+				trace_printk("USE FRAME POINTER\n");
+				return 0; }
 			continue;
 		default:
 			WARN_ONCE(1, "Undefined unwind bit %d", bit);

And have this:

# trace-cmd show
[..]
       trace-cmd-1136    [001] .....   239.482498: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482501: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482504: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482507: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482821: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482825: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482828: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482831: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.482834: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483004: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483007: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483010: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483013: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483016: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483230: unwind_user: USE FRAME POINTER
       trace-cmd-1136    [001] .....   239.483234: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483238: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483240: unwind_user: USE EH_FRAME
       trace-cmd-1136    [001] .....   239.483243: unwind_user: USE EH_FRAME
[..]

Note, the compile had one warning (with all patches applied):

  vmlinux.o: warning: objtool: eh_frame_find+0x270c: call to eh_frame_do_expression.isra.0() with UACCESS enabled

I'll try to get time when I get back from my travels next week to look
at each of the patches.

I also do not think this makes sframe obsolete. I believe there's holes
with eh_frame and some may not like the complexity of it. But this
gives us an honest look at what options are available to us.

Thanks for doing this!

-- Steve