Did the verification for the kernel (linux-generic) like above and based
on representative kernel modules (arch-specific and common):
noble:
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 24.04 LTS
Release: 24.04
Codename: noble
$ uname -a
Linux testlpar1 6.8.0-35-generic #35-Ubuntu SMP Mon May 20 15:36:54 UTC 2024
s390x s390x s390x GNU/Linux
$ sudo unzstd
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/arch/s390/crypto/aes_s390.ko.zst
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/arch/s390/crypto/aes_s390.ko.zst:
507329 bytes
$ readelf -wi
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/arch/s390/crypto/aes_s390.ko
| grep DW_AT_produce | grep -c mbackchain
2
$ readelf -wi
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/arch/s390/crypto/aes_s390.ko
| grep DW_AT_produce | grep -c no-omit-frame-pointer
0
$ sudo unzstd
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/fs/nfs/nfsv4.ko.zst
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/fs/nfs/nfsv4.ko.zst:
17383593 bytes
$ readelf -wi
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/fs/nfs/nfsv4.ko | grep
DW_AT_produce | grep -c mbackchain
24
$ readelf -wi
/usr/lib/debug/lib/modules/6.8.0-35-generic/kernel/fs/nfs/nfsv4.ko | grep
DW_AT_produce | grep -c no-omit-frame-pointer
0
oracular:
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu Oracular Oriole (development branch)
Release: 24.10
Codename: oracular
$ uname -a
Linux s1lp14 6.11.0-4-generic #4-Ubuntu SMP Tue Aug 20 14:03:40 UTC 2024 s390x
s390x s390x GNU/Linux
$ sudo unzstd
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/s390/crypto/aes_s390.ko.zst
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/s390/crypto/aes_s390.ko.zst:
529441 bytes
$ readelf -wi
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/s390/crypto/aes_s390.ko
| grep DW_AT_produce | grep -c mbackchain
2
$ readelf -wi
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/s390/crypto/aes_s390.ko
| grep DW_AT_produce | grep -c no-omit-frame-pointer
0
$ sudo unzstd
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko.zst
[sudo] password for ubuntu:
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko.zst:
18990945 bytes
$ readelf -wi
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko | grep
DW_AT_produce | grep -c mbackchain
24
$ readelf -wi
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko | grep
DW_AT_produce | grep -c no-omit-frame-pointer
0
Like one can see '-mbackchain' is set, but "-no-omit-frame-pointer" is
not set, hence successful verified.
Closing the affected 'linux (Ubuntu)' as 'Fix Released' - and with that
the project entry as well.
** Changed in: linux (Ubuntu Noble)
Status: New => Fix Released
** Changed in: linux (Ubuntu Oracular)
Status: New => Fix Released
** Changed in: ubuntu-z-systems
Status: New => Fix Released
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Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/2064538
Title:
Revert back frame pointers for s390x (remove -fno-omit-frame-pointer
but use -mbackchain)
Status in Ubuntu on IBM z Systems:
Fix Released
Status in dpkg package in Ubuntu:
Fix Released
Status in glibc package in Ubuntu:
Fix Released
Status in linux package in Ubuntu:
Fix Released
Status in dpkg source package in Noble:
Fix Released
Status in glibc source package in Noble:
Fix Released
Status in linux source package in Noble:
Fix Released
Status in dpkg source package in Oracular:
Fix Released
Status in glibc source package in Oracular:
Fix Released
Status in linux source package in Oracular:
Fix Released
Bug description:
SRU Justification:
[ Impact ]
* The preferred way of doing stack unwinding on Linux on Z is via dwarf call
frame information.
In absence of a dwarf unwinder (as in the Linux kernel) a stack chain can be
maintained at runtime in addition to the dwarf unwinding information.
* This allows for simple backtrace implementations, but imposes a
small runtime overhead. For this to work, all code that might be part
of backtrace must be built with the -mbackchain GCC option.
* The -fno-omit-framepointer switch is neither necessary nor helpful in this
context.
Having a (soft/simulated) frame pointer does not improve backtraces at all
on IBM Z.
* However, forcing a frame pointer via the -fno-omit-frame-pointer
option negatively affects performance for multiple reasons: extra
prologue/epilogue overhead and fewer shrink-wrapping opportunities.
* Given -fno-omit-frame-pointer does not provide any improvements
(backtraces or otherwise) and only reduces performance, -fno-omit-
frame-pointers should not be used on IBM Z.
* So we are facing here a performance penalty without any gain - on
this particular platform.
* And sometimes (in rare cases like LP#2060108) frame pointers may
even lead to failed builds.
[ Test Plan ]
* Due to the above description of the impact and rationale,
this pragmatic approach for testing is given:
* Build the affected packages where frame-pointers should be reverted
using the updated dpkg package (that incl. the modified build defaults)
on (or for) this particular platform.
* Now frame-pointer usage be checked in the following different ways:
* 1) For the ease of use (and thanks to Julian Klode), there is this python
test script available that allows to verify a binary in regard to
frame pointers:
https://gist.github.com/julian-klode/85e55553f85c410a1b856a93dce77208
https://gist.githubusercontent.com/julian-klode/85e55553f85c410a1b856a93dce77208/raw/488b8509e6f23fe48f917961fe711b285dcb2e28/dwprod.py
requires python3-pyelftools
* 2) Another more manual way is to verify based on debug symbols like this:
- find and install the ddeb package
- maybe extract the file (e.g. unzstd)
- use 'readelf -wi'
- and grep for 'DW_AT_produce' (build options)
- look for entries regarding frame-pointer
The output may look similar to this:
readelf -wi
./usr/lib/debug/lib/modules/6.8.0-38-generic/kernel/arch/s390/crypto/aes_s390.ko
| grep DW_AT_produce
<23> DW_AT_producer : (indirect string, offset: 0x7d): GNU AS
2.42
<129> DW_AT_producer : (indirect string, offset: 0x3eef): GNU
C11 13.2.0 -m64 -mpacked-stack -mbackchain -msoft-float -march=z13 -mtune=z16
-mindirect-branch=thunk-extern -mfunction-return=thunk-extern
-mindirect-branch-table -mrecord-mcount -mnop-mcount -mfentry -mzarch -g
-gdwarf-5 -O2 -std=gnu11 -p -fshort-wchar -funsigned-char -fno-common
-fno-strict-aliasing -fno-asynchronous-unwind-tables
-fno-delete-null-pointer-checks -fno-allow-store-data-races
-fno-stack-protector -ftrivial-auto-var-init=zero -fno-stack-clash-protection
-fzero-call-used-regs=used-gpr -fno-inline-functions-called-once
-falign-functions=8 -fstrict-flex-arrays=3 -fno-strict-overflow
-fstack-check=no -fconserve-stack -fsanitize=bounds-strict -fsanitize=shift
-fsanitize=bool -fsanitize=enum -fPIC
* 3) And maybe watching the build messages / log for the build options that
were used (but that is probably not sufficient - it's better to inspect
the output.)
[ Where problems could occur ]
* The dpkg modifications could have been done erroneously.
A dpkg test build and/or builds of other packages with the modified dpkg
version in place would show this.
* The settings in dpkg might be overwritten by other settings/packages.
Tests like above, would show this.
* One may think there could be issues in an environment where some packages
have frame-pointer enabled and other don't.
This is fine and was confirmed by IBM toolchain team and ours
(as well as by a longer running <weeks> test system,
with FP disabled in kernel, that showed no issues - like expected).
[ Other Info ]
* These changes were implemented during the opening of the oracular series.
The very same changes are backported to 24.04 LTS.
* These only affect the ppc64el and s390x architectures,
for other architectures it's a no-change upload.
* We didn't see any fallout for these changes during the development
on the oracular series, and therefore don't expect any fallout or
regressions in 24.04 LTS either.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu-z-systems/+bug/2064538/+subscriptions
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