Handling pgoff in perf elf mmap/mmap2 elf info

Milian Wolff mail@milianw.de
Wed Oct 17 14:52:00 GMT 2018


On Montag, 15. Oktober 2018 23:06:07 CEST Milian Wolff wrote:
> On Montag, 15. Oktober 2018 23:04:52 CEST Mark Wielaard wrote:
> > Hi Milian,
> > 
> > On Mon, 2018-10-15 at 22:38 +0200, Milian Wolff wrote:
> > > here's one example of mmap events recorded by perf:
> > > 
> > > 0x7fac5ec0b000 to 0x7fac5ed9a000, len =   0x18f000, offset
> > > =                0    r--p    /usr/lib/libstdc++.so.6.0.25
> > > 0x7fac5ec94000 to 0x7fac5ed8a000, len =    0xf6000, offset
> > > =          0x89000    ---p    /usr/lib/libstdc++.so.6.0.25
> > > 0x7fac5ec94000 to 0x7fac5ed4c000, len =    0xb8000, offset
> > > =          0x89000    r-xp    /usr/lib/libstdc++.so.6.0.25
> > > 0x7fac5ed4c000 to 0x7fac5ed89000, len =    0x3d000, offset
> > > =         0x141000    r--p    /usr/lib/libstdc++.so.6.0.25
> > > 0x7fac5ed8a000 to 0x7fac5ed97000, len =     0xd000, offset
> > > =         0x17e000    rw-p    /usr/lib/libstdc++.so.6.0.25
> > 
> > Could you also post the matching phdr output for the file?
> > eu-readelf -l /usr/lib/libstdc++.so.6.0.25 should show it.
> > That way we can see how the PT_LOAD segments map to the mmap events.
> 
> Sure:
> 
> $ eu-readelf -l /usr/lib/libstdc++.so.6.0.25
> Program Headers:
>   Type           Offset   VirtAddr           PhysAddr           FileSiz
> MemSiz   Flg Align
>   LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x088fa8
> 0x088fa8 R   0x1000
>   LOAD           0x089000 0x0000000000089000 0x0000000000089000 0x0b7ae1
> 0x0b7ae1 R E 0x1000
>   LOAD           0x141000 0x0000000000141000 0x0000000000141000 0x03cfe0
> 0x03cfe0 R   0x1000
>   LOAD           0x17e8e0 0x000000000017f8e0 0x000000000017f8e0 0x00b8b8
> 0x00ed60 RW  0x1000
>   DYNAMIC        0x1873a8 0x00000000001883a8 0x00000000001883a8 0x0001e0
> 0x0001e0 RW  0x8
>   NOTE           0x0002a8 0x00000000000002a8 0x00000000000002a8 0x000024
> 0x000024 R   0x4
>   NOTE           0x17dfc0 0x000000000017dfc0 0x000000000017dfc0 0x000020
> 0x000020 R   0x8
>   TLS            0x17e8e0 0x000000000017f8e0 0x000000000017f8e0 0x000000
> 0x000020 R   0x8
>   GNU_EH_FRAME   0x149558 0x0000000000149558 0x0000000000149558 0x007f04
> 0x007f04 R   0x4
>   GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000
> 0x000000 RW  0x10
>   GNU_RELRO      0x17e8e0 0x000000000017f8e0 0x000000000017f8e0 0x00b720
> 0x00b720 R   0x1
> 
>  Section to Segment mapping:
>   Segment Sections...
>    00      [RO: .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version
> .gnu.version_d .gnu.version_r .rela.dyn]
>    01      [RO: .init .text .fini]
>    02      [RO: .rodata .eh_frame_hdr .eh_frame .gcc_except_table
> .note.gnu.property]
>    03      [RELRO: .tbss .init_array .fini_array .data.rel.ro .dynamic .got]
> .got.plt .data .bss
>    04      [RELRO: .dynamic]
>    05      [RO: .note.gnu.build-id]
>    06      [RO: .note.gnu.property]
>    07      [RELRO: .tbss]
>    08      [RO: .eh_frame_hdr]
>    09
>    10      [RELRO: .tbss .init_array .fini_array .data.rel.ro .dynamic .got]

So, Mark - any chance you could have a look at the above and give us your 
feedback?

When I compare the actual mmap events with the LOAD segments, there are some 
similarities, but also some discrepancies. Note how the mmap sizes always 
differ from the FileSiz header value. And the offsets also sometimes mismatch, 
e.g. for the last segment / mmap event we get 0x17f8e0 in the header, but 
0x17e000 in the mmap event...:

LOAD           0x17e8e0 0x000000000017f8e0 0x000000000017f8e0 0x00b8b8
 0x00ed60 RW  0x1000

0x7fac5ed8a000 to 0x7fac5ed97000, len =     0xd000, offset =         0x17e000    
rw-p    /usr/lib/libstdc++.so.6.0.25

I'm pretty confused here!

-- 
Milian Wolff
mail@milianw.de
http://milianw.de
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