This bug was fixed in the package glibc - 2.40-1ubuntu1
---------------
glibc (2.40-1ubuntu1) oracular; urgency=medium
* Merge 2.40-1 from Debian experimental (LP: #2073372)
Dropped changes:
- d/p/ubuntu/lv_LV-currency.diff: applied upstream
- d/p/ubuntu/apparmor/0001-*: applied upstream
Refreshed patches:
- d/p/ubuntu/fix-fortify-source.patch: adjusted to take new code into
account
* debian/patches/git-updates.diff: update from upstream stable branch
-- Simon Chopin <scho...@ubuntu.com> Fri, 26 Jul 2024 11:16:06 +0200
** Changed in: glibc (Ubuntu Oracular)
Status: Fix Committed => Fix Released
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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:
New
Status in dpkg package in Ubuntu:
Fix Released
Status in glibc package in Ubuntu:
Fix Released
Status in linux package in Ubuntu:
New
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:
New
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:
New
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
* 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.
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