Handling pgoff in perf elf mmap/mmap2 elf info

Milian Wolff mail@milianw.de
Mon Oct 15 20:39:00 GMT 2018

On Donnerstag, 11. Oktober 2018 20:14:43 CEST Milian Wolff wrote:
> On Donnerstag, 11. Oktober 2018 19:37:07 CEST Mark Wielaard wrote:
> > Hi,
> > 
> > My apologies for not having looked deeper at this.
> > It is a bit tricky and I just didnt have enough time to
> > really sit down and think it all through yet.
> > 
> > On Thu, Oct 11, 2018 at 05:02:18PM +0000, Ulf Hermann wrote:
> > > is there any pattern in how the loader maps the ELF sections into
> > > memory? What sections does it actually map and which of those do we need
> > > for unwinding?
> > 
> > Yes, it would be helpful to have some examples of mmap events plus
> > the associated segment header (eu-readelf -l) of the ELF file.
> > 
> > Note that the kernel and dynamic loader will use the (PT_LOAD) segments,
> > not the sections, to map things into memory. Each segment might contain
> > multiple sections.
> > 
> > libdwfl then tries to associate the correct sections (and address bias)
> > with how the ELF file was mapped into memory.
> > 
> > > I hope that only one of those MMAPs per ELF is actually meaningful and
> > > we can simply add that one's pgoff as an extra member to Dwfl_Module and
> > > use it whenever we poke the underlying file.
> > 
> > One "trick" might be to just substract the pgoff from the load address.
> > And so report as if the ELF file was being mapped from the start. This
> > isn't really correct, but it might be interesting to see if that makes
> > libdwfl able to just associate the whole ELF file with the correct
> > address map.
> I'll try to come up with some minimal code examples we can use to test all
> of this. But from what I remember, neither of the above suggestions will be
> sufficient as we can still run into overlapping module errors from elfutils
> when we always load everything. I.e. I believe we've seen mappings that
> eventually become partially obsoleted by a future mmap event. At that
> point, we somehow need to be able to only map parts of a file, not all of
> it. So just subtracting or honoring pgoff is not enough, I believe we also
> need to be able to explicitly say how much of a file to map.
> But to make this discussion easier to follow for others, I'll create some
> standalone cpp code that takes a `perf script --show-mmap-events  | grep
> PERF_RECORD_MMAP` input file and then runs this through elfutils API to
> reproduce the issues we are facing.
> I'll get back to you all once this is done.

Hey all,

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

this is noteworthy in multiple ways:

- the first mapping we receive is for pgoff = 0 for the full file size aligned 
to the page boundary
- the first mapping isn't executable yet
- the last mappings have a huge offset which actually lies beyond the 
initially mmaped region?!

And to make things worse, when we report the file at address 0x7fac5ec0b000 
via dwfl, we get:

reported module /usr/lib/libstdc++.so.6.0.25
        expected: 0x7fac5ec0b000 to 0x7fac5ed9a000 (0x18f000)
        actual:   0x7fac5ec0b000 to 0x7fac5ed99640 (0x18e640)

So now dwfl won't ever be able to map any addresses into this module when they 
come after 0x7fac5ed99640, but the mmap events above seem to indicate that 
this could be possible?

I'll now upload my code to enable you all to play around with this yourself.

Milian Wolff
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