Re: [PATCH 0/8] unwind, arm64: add sframe unwinder for kernel

Weinan Liu
2025-02-25T01:02:31Z

On Mon, Feb 10, 2025 at 12:30 AM Weinan Liu <wnliu@google.com> wrote:
> I already have a WIP patch to add sframe support to the kernel module.
> However, it is not yet working. I had trouble unwinding frames for the
> kernel module using the current algorithm.
>
> Indu has likely identified the issue and will be addressing it from the
> toolchain side.
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=32666

I have a working in progress patch that adds sframe support for kernel
module.
https://github.com/heuza/linux/tree/sframe_unwinder.rfc

According to the sframe table values I got during runtime testing, looks
like the offsets are not correct .

When unwind symbols init_module(0xffff80007b155048) from the kernel
module(livepatch-sample.ko), the start_address of the FDE entries in the
sframe table of the kernel modules appear incorrect.
For instance, the first FDE's start_addr is reported as -20564. Adding
this offset to the module's sframe section address (0xffff80007b15a040)
yields 0xffff80007b154fec, which is not within the livepatch-sample.ko
memory region(It should be larger than 0xffff80007b155000).

Here are the sframe table values of the livepatch-samples.ko that I print
by qemu + gdb.

```
$ /usr/bin/aarch64-linux-gnu-objdump -L --sframe=.sframe ./samples/livepatch/livepatch-sample.ko
./samples/livepatch/livepatch-sample.ko:     file format elf64-littleaarch64

Contents of the SFrame section .sframe:
Header :

Version: SFRAME_VERSION_2
Flags: SFRAME_F_FDE_SORTED
Num FDEs: 3
Num FREs: 11

Function Index :

func idx [0]: pc = 0x0, size = 12 bytes
STARTPC         CFA       FP        RA
0000000000000000  sp+0      u         u

func idx [1]: pc = 0x0, size = 44 bytes
STARTPC         CFA       FP        RA
0000000000000000  sp+0      u         u
000000000000000c  sp+0      u         u[s]
0000000000000010  sp+16     c-16      c-8[s]
0000000000000024  sp+0      u         u[s]
0000000000000028  sp+0      u         u

func idx [2]: pc = 0x0, size = 56 bytes
STARTPC         CFA       FP        RA
0000000000000000  sp+0      u         u
000000000000000c  sp+0      u         u[s]
0000000000000010  sp+16     c-16      c-8[s]
0000000000000030  sp+0      u         u[s]
0000000000000034  sp+0      u         u

(gdb) bt
#0  find_fde (tbl=0xffff80007b157708, pc=18446603338286190664) at kernel/sframe_lookup.c:75
#1  0xffff80008031e260 in sframe_find_pc (pc=18446603338286190664, entry=0xffff800086f83800) at kernel/sframe_lookup.c:175
#2  0xffff800080035a48 in unwind_next_frame_sframe (state=0xffff800086f83828) at arch/arm64/kernel/stacktrace.c:270
#3  kunwind_next (state=0xffff800086f83828) at arch/arm64/kernel/stacktrace.c:332
...

(gdb) lx-symbols
loading vmlinux
scanning for modules in /home/wnliu/kernel
loading @0xffff80007b155000: /home/wnliu/kernel/samples/livepatch/livepatch-sample.ko
loading @0xffff80007b14d000: /home/wnliu/kernel/fs/fat/vfat.ko
loading @0xffff80007b130000: /home/wnliu/kernel/fs/fat/fat.ko

(gdb) p/x *tbl->sfhdr_p
$5 = {preamble = {magic = 0xdee2, version = 0x2, flags = 0x1}, abi_arch = 0x2, cfa_fixed_fp_offset = 0x0, cfa_fixed_ra_offset = 0x0, auxhdr_len = 0x0, num_fdes = 0x3, num_fres = 0xb, fre_len = 0x25, fdes_off = 0x0, fres_off = 0x3c}

(gdb) p/x tbl->sfhdr_p
$6 = 0xffff80007b15a040

(gdb) p *tbl->fde_p
$7 = {start_addr = -20564, size = 12, fres_off = 0, fres_num = 1, info = 0 '\000', rep_size = 0 '\000', padding = 0}

(gdb) p *(tbl->fde_p + 1)
$11 = {start_addr = -20552, size = 44, fres_off = 3, fres_num = 5, info = 0 '\000', rep_size = 0 '\000', padding = 0}

(gdb) p *(tbl->fde_p + 2)
$12 = {start_addr = -20508, size = 56, fres_off = 20, fres_num = 5, info = 0 '\000', rep_size = 0 '\000', padding = 0}

/* -20564 + 0xffff80007b15a040 = 0xffff80007b154fec */
(gdb) info symbol 0xffff80007b154fec
No symbol matches 0xffff80007b154fec
```