[apparmor] [Bug 2025030] Re: apparmor_parser -O no-expr-simplify problematic

[Michael Vogt]

I did a quick experiment in our "interfaces-many-snap-provided" test to
see what impact the options have. I ran spread with:

$ spread -shell-after -v qemu:ubuntu-22.04-64:tests/main/interfaces-
many-snap-provided

And inside spread (on 22.04) compiled the profiles with/without no-expr-
simplify:

With "no-expr-simplify"
# /usr/bin/time -f '%E real, %M max-mem'  apparmor_parser -O no-expr-simplify -S /var/lib/snapd/apparmor/profiles/snap.{test-snapd-policy-app-consumer,test-snapd-policy-app-provider-classic}.* >/dev/null
0:22.07 real, 22908 max-mem

Without "no-expr-simplify"
# /usr/bin/time -f '%E real, %M max-mem'  apparmor_parser -S /var/lib/snapd/apparmor/profiles/snap.{test-snapd-policy-app-consumer,test-snapd-policy-app-provider-classic}.* >/dev/null
0:49.45 real, 13720 max-mem

Which seems to indicate that there is a trade-off here and moving away
from "no-expr-simplify" will make the installs/refreshes slower but also
reduces memory usage. It also seems like our normal tests do not catch
the memory explosion that we saw here.

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Title:
  apparmor_parser -O no-expr-simplify problematic

Status in snapd:
  New

Bug description:
  There was a recent issue with a core refresh that caused breakage.
  Upon further investigation it turns out that the apparmor_parser uses
  an substantial of memory.

  Upon some more investigation it turns out that that -O no-expr-
  simplify makes both time to compile and memory usage increase 10x.
  Tested with 22.04 but I see the same ballpark results with 16.04:

  $ /usr/bin/time --verbose apparmor_parser -S 2.59/profiles/snap.screenly-client.command-executor > /dev/null
      Command being timed: "apparmor_parser -S 2.59/profiles/snap.screenly-client.command-executor"
      User time (seconds): 4.32
      Maximum resident set size (kbytes): 117392

  $ /usr/bin/time --verbose apparmor_parser  -O no-expr-simplify -S 2.59/profiles/snap.screenly-client.command-executor > /dev/null
      Command being timed: "apparmor_parser -O no-expr-simplify -S 2.59/profiles/snap.screenly-client.command-executor"
      User time (seconds): 40.64
      Maximum resident set size (kbytes): 1015816

  Profile is attached.

  
  It seems like we seriously need to consider dropping "-O no-expr-simplify". 

  For context:
  https://bugs.launchpad.net/ubuntu-rtm/+source/apparmor/+bug/1383858
  is why it was added in the first place

  And some recent work to make things faster:
  https://gitlab.com/apparmor/apparmor/-/merge_requests/711

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