On Montag, 11. Januar 2021 17:05:30 CET Mark Wielaard wrote: > Hi Milian, > > On Wed, 2020-12-30 at 16:51 +0100, Milian Wolff wrote: > > I stumbled upon a strange behavior while symbolizing a perf.data file > > with > > hotspot/perfparser/elfutils that I have trouble understanding. I hope it's > > ok to send this call for help here. > > > > I'm running with elfutils 0.181, g++ 10.2, glibc 2.32 on archlinux with > > kernel 5.9.14. > > [...] > > So here, the address `0x555555555a40` should come from the second mapping, > > which has an offset 0x0 and starts at `0x555555555000` and has a size of > > `0x1000`. How can that possibly map to the `main` symbol which has an > > offset of `0x1a40`? > > > > Does anyone know what's going on here? > > Isn't this simply the ELF file being loaded/mmapped in separate > (overlapping) chunks?
Ah, indeed - thanks for this question! It actually seems like perf only
records this one single mmap event. I.e.:
```
$ perf record ./a.out
$ perf script --show-mmap-events | grep -E 'MMAP.*a.out'
a.out 139334 16992.223215: PERF_RECORD_MMAP2 139334/139334:
[0x55bb21e70000(0x1000) @ 0 fe:00 8009266 4286144336]: r-xp /home/milian/
projects/kdab/rnd/hotspot/build/a.out
```
And that is probably the bug. Because when I run the following slightly
changed perf session, the issue doesn't occur - and I see more mmap events
too:
```
$ perf record --call-graph dwarf ./a.out
$ perf script --show-mmap-events 2>/dev/null | grep -E 'MMAP.*a.out'
a.out 139453 17065.223302: PERF_RECORD_MMAP2 139453/139453:
[0x5589a3711000(0x5000) @ 0 fe:00 8009266 4286144336]: r--p /home/milian/
projects/kdab/rnd/hotspot/build/a.out
a.out 139453 17065.223305: PERF_RECORD_MMAP2 139453/139453:
[0x5589a3712000(0x1000) @ 0 fe:00 8009266 4286144336]: r-xp /home/milian/
projects/kdab/rnd/hotspot/build/a.out
a.out 139453 17065.223306: PERF_RECORD_MMAP2 139453/139453:
[0x5589a3713000(0x2000) @ 0 fe:00 8009266 4286144336]: rw-p /home/milian/
projects/kdab/rnd/hotspot/build/a.out
a.out 139453 17065.223307: PERF_RECORD_MMAP2 139453/139453:
[0x5589a3715000(0x1000) @ 0x1000 fe:00 8009266 4286144336]: rw-p /home/milian/
projects/kdab/rnd/hotspot/build/a.out
a.out 139453 17065.223994: PERF_RECORD_MMAP2 139453/139453:
[0x5589a3713000(0x2000) @ 0 fe:00 8009266 4286144336]: r--p /home/milian/
projects/kdab/rnd/hotspot/build/a.out
```
Thank you Mark, I'll take this over to the perf mailing list, as it doesn't
seem to be a problem with elfutils.
> What does eu-readelf -l show?
Probably irrelevant now, but here is the output nevertheless:
```
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz
MemSiz Flg Align
PHDR 0x000040 0x0000000000000040 0x0000000000000040 0x000268
0x000268 R 0x8
INTERP 0x0002a8 0x00000000000002a8 0x00000000000002a8 0x00001c
0x00001c R 0x1
[Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000934
0x000934 R 0x1000
LOAD 0x000940 0x0000000000001940 0x0000000000001940 0x000530
0x000530 R E 0x1000
LOAD 0x000e70 0x0000000000002e70 0x0000000000002e70 0x000218
0x000218 RW 0x1000
LOAD 0x001088 0x0000000000004088 0x0000000000004088 0x000050
0x0001c8 RW 0x1000
DYNAMIC 0x000e88 0x0000000000002e88 0x0000000000002e88 0x0001d0
0x0001d0 RW 0x8
GNU_RELRO 0x000e70 0x0000000000002e70 0x0000000000002e70 0x000218
0x001190 R 0x1
GNU_EH_FRAME 0x000828 0x0000000000000828 0x0000000000000828 0x000034
0x000034 R 0x4
GNU_STACK 0x000000 0x0000000000000000 0x0000000000000000 0x000000
0x000000 RW 0x0
NOTE 0x0002c4 0x00000000000002c4 0x00000000000002c4 0x000038
0x000038 R 0x4
Section to Segment mapping:
Segment Sections...
00
01 [RO: .interp]
02 [RO: .interp .note.ABI-tag .note.gnu.build-id .dynsym .gnu.version
.gnu.version_r .gnu.hash .dynstr .rela.dyn .rela.plt .rodata .eh_frame_hdr
.eh_frame]
03 [RO: .text .init .fini .plt]
04 [RELRO: .fini_array .init_array .dynamic .got]
05 .data .got.plt .bss
06 [RELRO: .dynamic]
07 [RELRO: .fini_array .init_array .dynamic .got]
08 [RO: .eh_frame_hdr]
09
10 [RO: .note.ABI-tag .note.gnu.build-id]
```
> Is the address/offset range loaded/mmapped executable?
See above, I now believe the problem is the lack of reported mmap event to
begin with.
--
Milian Wolff
m...@milianw.de
http://milianw.de
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