hwpmc has been utterly broken for userspace binaries, and has been
labeling all samples from userspace binaries as dubious frames. The
issues are that:
-The check for ph.p_offset & (-ph.p_align) == 0 was mostly bogus. The
intent was to ignore all executable segments other than the first,
which when using BFD appeared in the first page, but with current LLD
a read-only data segment appears before the executable segment,
pushing the latter into the second page or later. This meant no
executable segment was ever found, and thus pi_vaddr remained
0. Instead of relying on BFD's layout, track whether we've seen an
executable segment explicitly with a local bool.
-Shared libraries were not parsing the segments to calculate pi_vaddr,
resulting in it always being 0. Again, when using BFD, the executable
segment started at the first page, and so pi_vaddr was genuinely
meant to be 0, but not with LLD's current layout. This meant that
pmcstat_image_link's offset calculation gave the base address of the
segment in memory, rather than the base address of the whole library
in memory, and so when adding that to pi_start/pi_end to get the
range of the executable sections in memory it double-counted the
offset of the first executable segment within the library. Thus we
need to do the exact same parsing for ET_DYN as we do for ET_EXEC,
which is simpler to write as special-casing ET_REL to not look for
segments. Note that, whilst PT_INTERP isn't needed for shared
libraries, it will be for PIEs, which pmcstat still fails to handle
due to not knowing the base address of the PIE; we get the base
address for libraries by MAP_IN events, and for rtld by virtue of the
process's entry address being rtld's, but have no equivalent for the
executable.
Fixes courtesy of jrtc27@.
Reviewed by: jrtc27, jhb (earlier version)
Differential Revision: https://reviews.freebsd.org/D33055
Sponsored by: Netflix
517a7adb11