Gentoo Websites Logo
Go to: Gentoo Home Documentation Forums Lists Bugs Planet Store Wiki Get Gentoo!

Bug 854738

Summary: app-misc/sphinx-2.2.11-r5 fails to compile (lto): sphinxudf.c:305:17: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
Product: Gentoo Linux Reporter: Agostino Sarubbo <ago>
Component: Current packagesAssignee: Hans de Graaff <graaff>
Status: CONFIRMED ---    
Severity: normal CC: koelblandreas
Priority: Normal    
Version: unspecified   
Hardware: All   
OS: Linux   
Whiteboard:
Package list:
Runtime testing required: ---
Bug Depends on:    
Bug Blocks: 618550    
Attachments: build.log
Patch that avoids undefined behaviour because of c++ reinterpret_cast

Description Agostino Sarubbo gentoo-dev 2022-06-28 06:44:21 UTC
https://blogs.gentoo.org/ago/2020/07/04/gentoo-tinderbox/

Issue: app-misc/sphinx-2.2.11-r5 fails to compile (lto).
Discovered on: amd64 (internal ref: lto_tinderbox)

NOTE:
This machine uses lto with CFLAGS=-flto -Werror=odr -Werror=lto-type-mismatch -Werror=strict-aliasing
Comment 1 Agostino Sarubbo gentoo-dev 2022-06-28 06:44:24 UTC
Created attachment 788249 [details]
build.log

build log and emerge --info
Comment 2 Agostino Sarubbo gentoo-dev 2022-06-28 06:51:34 UTC
I'm not 100% sure that it is an lto fault. After if you dig into it, please remove the blocker bug if you think this has nothing to do with lto. Thanks
Comment 3 Andreas K├Âlbl 2022-06-28 16:17:48 UTC
Created attachment 788513 [details, diff]
Patch that avoids undefined behaviour because of c++ reinterpret_cast
Comment 4 Agostino Sarubbo gentoo-dev 2022-07-26 09:00:25 UTC
Here is a bit of explanation:

-Werror=lto-type-mismatch:
User to find possible runtime issues in packages. It likely means the package is unsafe to build & use with LTO.
For projects using the same identifier but with different types across different files, they must be fixed to be consistent across the codebase.

-Werror=odr:
Used to find possible runtime issues in packages. These bugs are a problem anyway but may be even worse when combined with LTO. C++ code must comply with the One Definition Rule (ODR) - see https://en.cppreference.com/w/cpp/language/definition#One_Definition_Rule.

-Werror=strict-aliasing:
Used to find possible runtime issues in packages. These bugs are a problem anyway but may be even worse when combined with LTO.

Workarounds:
- If upstream is friendly and still active, file a bug upstream. For emulators, codecs, games, or multimedia packages, it may be worth just applying a workaround instead, as upstreams sometimes aren't receptive to these bugs (VALID FOR ALL).
- Use the new 'filter-lto' from flag-o-matic.eclass as it's likely to be unsafe with LTO (VALID FOR lto-type-mismatch - odr).
- Fix it yourself if interested, of course (VALID FOR ALL).
- Append-flags -fno-strict-aliasing (VALID FOR strict-aliasing).
- Use memcpy() but a union is sometimes suitable too (VALID FOR strict-aliasing).
- -fstrict-aliasing is implied by -O2, so this must be addressed in some form (VALID FOR strict-aliasing).

See also: https://marc.info/?l=gentoo-dev&m=165639574126280&w=2