On Mon, Jun 30, 2025 at 09:31:44AM +0200, Peter Zijlstra wrote: > On Fri, Jun 27, 2025 at 10:34:15AM -0700, Josh Poimboeuf wrote: > > On Fri, Jun 27, 2025 at 12:53:28PM +0200, Peter Zijlstra wrote: > > > On Thu, Jun 26, 2025 at 04:55:29PM -0700, Josh Poimboeuf wrote: > > > > An upcoming patch will add the SHF_MERGE flag to x86 __jump_table and > > > > __bug_table so their entry sizes can be defined in inline asm. > > > > > > > > However, those sections have SHF_WRITE, which the Clang linker (lld) > > > > explicitly forbids combining with SHF_MERGE. > > > > > > > > Those sections are modified at runtime and must remain writable. While > > > > SHF_WRITE is ignored by vmlinux, it's still needed for modules. > > > > > > > > To work around the linker interference, remove SHF_WRITE during > > > > compilation and restore it after linking the module. > > > > > > This is vile... but I'm not sure I have a better solution. > > > > > > Eventually we should get the toolchains fixed, but we can't very well > > > mandate clang-21+ to build x86 just yet. > > > > Yeah, I really hate this too. I really tried to find something better, > > including mucking with the linker script, but this was unfortunately the > > only thing that worked. > > > > Though, looking at it again, I realize we can localize the pain to Clang > > (and the makefile) by leaving the code untouched and instead strip > > SHF_WRITE before the link and re-add it afterwards. Then we can tie > > this horrible hack to specific Clang versions when it gets fixed. > > Oh yeah, that might be nicer indeed!
I ended up giving up on this approach. It was just too painful trying to get the toolchain to cooperate. It does kind of make sense that SHF_WRITE and SHF_MERGE are incompatible. What I really needed was a way to set entsize from assembly, but that doesn't seem possible without SHF_MERGE. For now I'm just going with marking the beginning of each special section entry with an objtool annotation. Then objtool can deduce the sizes from the distance between the annotations. -- Josh
