Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
On Wed, Jun 04, 2025 at 05:22:15PM -0700, Josh Poimboeuf wrote: > On Mon, May 26, 2025 at 01:06:34PM +0200, Peter Zijlstra wrote: > > On Fri, May 09, 2025 at 01:17:06PM -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. > > > > *groan* > > > > This and the following patches marking a whole bunch of sections M, > > seems to suggest you're going to rely on sh_entsize actually working. > > > > There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later > > if you want this to be so. > > Hm, ISTR this working with clang 18, I'll go test that again. You're right, looks like sh_entsize is getting cleared by the linker with my Clang 18. I guess I tested with newer Clang. "objtool klp diff" fails with: vmlinux.o: error: objtool: .discard.annotate_insn: unknown entry size So yeah, non-buggy linker is already being enforced, though I should probably make the error more human friendly. -- Josh
Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
On Mon, May 26, 2025 at 01:06:34PM +0200, Peter Zijlstra wrote: > On Fri, May 09, 2025 at 01:17:06PM -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. > > *groan* > > This and the following patches marking a whole bunch of sections M, > seems to suggest you're going to rely on sh_entsize actually working. > > There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later > if you want this to be so. Hm, ISTR this working with clang 18, I'll go test that again. -- Josh
Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
On Fri, May 09, 2025 at 01:17:06PM -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. *groan* This and the following patches marking a whole bunch of sections M, seems to suggest you're going to rely on sh_entsize actually working. There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later if you want this to be so.
