DW_AT_comp_dir and O= usage not working with objdump -dS, perf probe, etc
From: Arnaldo Carvalho de Melo
Date: Wed Apr 30 2025 - 19:34:48 EST
Hi,
I noticed recently while testing some other patches that
disassembling with objdump -dS didn't work when building the kernel with
O= as it sets it to the build dir, not to where the sources are, for
instance:
Make sure perf uses objdump to disassembly:
root@number:~# rm -f ~/.perfconfig
root@number:~# perf config annotate.disassemblers=objdump
root@number:~# perf record -a sleep 5
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 2.012 MB perf.data (6307 samples) ]
root@number:~# pahole --running_kernel_vmlinux
/lib/modules/6.15.0-rc4+/build/vmlinux
root@number:~# readelf -wi /lib/modules/6.15.0-rc4+/build/vmlinux | grep -m1 DW_AT_comp_dir
<17> DW_AT_comp_dir : (indirect line string, offset: 0): /home/acme/git/build/v6.15.0-rc4+
root@number:~#
root@number:~# readelf -wi /lib/modules/6.15.0-rc4+/build/vmlinux | grep -m1 DW_AT_comp_dir
<17> DW_AT_comp_dir : (indirect line string, offset: 0): /home/acme/git/build/v6.15.0-rc4+
root@number:~# perf report | grep -v ^# | head -10
16.53% swapper [kernel.kallsyms] [k] poll_idle
11.21% swapper [kernel.kallsyms] [k] acpi_os_read_port
1.39% swapper [kernel.kallsyms] [k] io_idle
0.92% swapper [kernel.kallsyms] [k] switch_mm_irqs_off
0.87% swapper [kernel.kallsyms] [k] __update_load_avg_cfs_rq
0.84% swapper [kernel.kallsyms] [k] read_tsc
0.76% swapper [kernel.kallsyms] [k] psi_group_change
0.76% swapper [kernel.kallsyms] [k] menu_select
0.67% swapper [kernel.kallsyms] [k] native_sched_clock
0.53% Isolated Web Co libxul.so [.] mozilla::EventListenerManager::AddEventListenerInternal(mozilla::dom::CallbackObjectHolder<mozilla::dom::EventListener, nsIDOMEventListener>, mozilla::EventMessage, nsAtom*, mozilla::EventListenerFlags const&, bool, bool, mozilla::dom::AbortSignal*)
root@number:~#
root@number:~# perf probe -L icmp_rcv
Failed to find source file path.
Error: Failed to show lines.
root@number:~# perf probe -v -L icmp_rcv
Looking at the vmlinux_path (8 entries long)
Using /lib/modules/6.15.0-rc4+/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/6.15.0-rc4+/build/vmlinux
fname: net/ipv4/icmp.c, lineno:1198
New line range: 1198 to 2147483647
path: net/ipv4/icmp.c
Search /home/acme/git/build/v6.15.0-rc4+/net/ipv4/icmp.c from debuginfod -> -2
Failed to find /home/acme/git/build/v6.15.0-rc4+/net/ipv4/icmp.c in debuginfod (d391f0e79126801bc8a8f907e763de7979941712)
Failed to find source file path.
Error: Failed to show lines. Reason: No such file or directory (Code: -2)
root@number:~#
In 'perf probe' we have the old --source that allows us to override that
DW_AT_comp_dir pointing to the build dir (O= one) and then it works:
root@number:~# perf probe --source /home/acme/git/linux/ -L icmp_rcv | head -20
<icmp_rcv@/home/acme/git/linux//net/ipv4/icmp.c:0>
0 int icmp_rcv(struct sk_buff *skb)
{
2 enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
struct rtable *rt = skb_rtable(skb);
struct net *net = dev_net_rcu(rt->dst.dev);
struct icmphdr *icmph;
if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) {
8 struct sec_path *sp = skb_sec_path(skb);
9 int nh;
if (!(sp && sp->xvec[sp->len - 1]->props.flags &
XFRM_STATE_ICMP)) {
reason = SKB_DROP_REASON_XFRM_POLICY;
goto drop;
}
17 if (!pskb_may_pull(skb, sizeof(*icmph) + sizeof(struct iphdr)))
goto drop;
root@number:~#
But objdump has no override and thus doesn't work:
root@number:~# objdump --disassemble=icmp_rcv -S /lib/modules/6.15.0-rc4+/build/vmlinux | head -40
/lib/modules/6.15.0-rc4+/build/vmlinux: file format elf64-x86-64
Disassembly of section .text:
ffffffff8231a6f0 <icmp_rcv>:
ffffffff8231a6f0: f3 0f 1e fa endbr64
ffffffff8231a6f4: e8 97 bf 05 ff call ffffffff81376690 <__fentry__>
ffffffff8231a6f9: 41 56 push %r14
ffffffff8231a6fb: 31 f6 xor %esi,%esi
ffffffff8231a6fd: 41 54 push %r12
ffffffff8231a6ff: 55 push %rbp
ffffffff8231a700: 53 push %rbx
ffffffff8231a701: 48 89 fb mov %rdi,%rbx
ffffffff8231a704: 48 83 ec 10 sub $0x10,%rsp
ffffffff8231a708: 48 8b 6f 58 mov 0x58(%rdi),%rbp
ffffffff8231a70c: 48 83 e5 fe and $0xfffffffffffffffe,%rbp
ffffffff8231a710: 48 8b 45 00 mov 0x0(%rbp),%rax
ffffffff8231a714: 4c 8b a0 08 01 00 00 mov 0x108(%rax),%r12
ffffffff8231a71b: e8 d0 e4 ff ff call ffffffff82318bf0 <__xfrm_policy_check2.constprop.0>
ffffffff8231a720: 85 c0 test %eax,%eax
ffffffff8231a722: 0f 85 c9 00 00 00 jne ffffffff8231a7f1 <icmp_rcv+0x101>
ffffffff8231a728: f6 43 7f 02 testb $0x2,0x7f(%rbx)
ffffffff8231a72c: 0f 84 bc 01 00 00 je ffffffff8231a8ee <icmp_rcv+0x1fe>
ffffffff8231a732: 48 8b 83 e0 00 00 00 mov 0xe0(%rbx),%rax
ffffffff8231a739: 0f b6 50 05 movzbl 0x5(%rax),%edx
ffffffff8231a73d: c1 e2 03 shl $0x3,%edx
ffffffff8231a740: 48 63 d2 movslq %edx,%rdx
ffffffff8231a743: 48 01 c2 add %rax,%rdx
ffffffff8231a746: 0f 84 a2 01 00 00 je ffffffff8231a8ee <icmp_rcv+0x1fe>
ffffffff8231a74c: 8b 02 mov (%rdx),%eax
ffffffff8231a74e: 83 e8 01 sub $0x1,%eax
ffffffff8231a751: 48 98 cltq
ffffffff8231a753: 48 83 f8 06 cmp $0x6,%rax
ffffffff8231a757: 0f 83 3b 03 00 00 jae ffffffff8231aa98 <icmp_rcv+0x3a8>
ffffffff8231a75d: 48 8b 44 c2 10 mov 0x10(%rdx,%rax,8),%rax
ffffffff8231a762: f6 80 01 01 00 00 10 testb $0x10,0x101(%rax)
ffffffff8231a769: 0f 84 7f 01 00 00 je ffffffff8231a8ee <icmp_rcv+0x1fe>
ffffffff8231a76f: 8b 53 70 mov 0x70(%rbx),%edx
root@number:~#
I haven't checked, ran out of time today, but I think this may be
involved:
commit 97282e6d380db8a07120fe1b794ac969ee4a3b5c
Author: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
Date: Sat Mar 22 10:03:16 2025 +0100
x86: drop unnecessary prefix map configuration
wdyt?
I'll continue tomorrow,
Cheers,
- Arnaldo