On Wed, Jan 29, 2025 at 04:02:34PM -0800, Andrii Nakryiko wrote:

On Tue, Jan 28, 2025 at 6:02 PM Josh Poimboeuf [off-list ref] wrote:
I'm not sure about this chunked lookup approach for arbitrary user
space applications. Those executable sections can be a) big and b)
discontiguous. E.g., one of the production binaries I looked at. Here
are its three main executable sections:

...
  [17] .bolt.org.text    PROGBITS         000000000b00e640  0ae0d640
       0000000011ad621c  0000000000000000  AX       0     0     64
...
  [48] .text             PROGBITS         000000001e600000  1ce00000
       0000000000775dd8  0000000000000000  AX       0     0     2097152
  [49] .text.cold        PROGBITS         000000001ed75e00  1d575e00
       00000000007d3271  0000000000000000  AX       0     0     64
...

Total text size is about 300MB:

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0x0000000000775dd8 + 0x00000000007d3271 + 0x0000000011ad621c

312603237

Section #17 ends at:

quoted

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hex(0x0000000011ad621c + 0x000000000b00e640)

'0x1cae485c'

While .text starts at 000000001e600000, so we have a gap of ~28MB:

quoted

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0x000000001e600000 - 0x1cae485c

28424100

So unless we do something more clever to support multiple
discontiguous chunks, this seems like a bad fit for user space.

Nothing clever needed, we could just have multiple sframe sections, each
one with a pointer to its text segment.  That would also have the
benefit of allowing the sframe data to be much more compact for the
noncontiguous cases.

I think having all this just binary searchable is already a big win
anyways and should be plenty fast, no?

Sframe is trying to compete with frame pointers which are MUCH faster.
3-4x faster in my testing, not including the page faults (which tend to
only affect performance in the very beginning).

-- 
Josh