Bug 22483 – DMD generates invalid string sections that work by coincidence

See https://github.com/rui314/mold/issues/126 DMD generates .rodata.str1.1 string sections (for char[] strings) with a sh_entsize of 0, when it should be 1. Linkers apparently react to this by not treating the section as mergeable at all. You can confirm this: ``` echo 'string foo() { return "Hello World"; }' > a.d echo 'string bar() { return "Hello World"; }' > b.d echo 'import a, b; void main() { assert(foo.ptr is bar.ptr); }' > c.d # force creating separate string sections dmd a.d -c -ofa.o dmd b.d -c -ofb.o dmd a.o b.o c.d -ofstrsectiontest # will fail ./strsectiontest ``` Patch: ``` diff --git a/src/dmd/backend/elfobj.d b/src/dmd/backend/elfobj.d index b555b01e0..02e888e8d 100644 --- a/src/dmd/backend/elfobj.d +++ b/src/dmd/backend/elfobj.d @@ -630,6 +630,7 @@ int ElfObj_string_literal_segment(uint sz) const int i = (sz == 4) ? 2 : sz - 1; const IDXSEC seg = ElfObj_getsegment(".rodata.str".ptr, name[i].ptr, SHT_PROGBITS, SHF_ALLOC | SHF_MERGE | SHF_STRINGS, sz); + MAP_SEG2SEC(seg).sh_entsize = sz; return seg; } ``` But with this applied, DMD generates invalid revocations: /usr/bin/ld: a.o: access beyond end of merged section (-4) /usr/bin/ld: b.o: access beyond end of merged section (-4) /usr/bin/ld: strsectiontest.o: access beyond end of merged section (-4) Afaics, the revocations were always invalid, but section merging didn't happen for the .rodata.str, so we didn't notice. How to fix that is beyond me though.

relocations*

@ssvb created dlang/dmd pull request #15915 "fix Issue 22483 - DMD generates invalid string sections that work by coincidence" fixing this issue: - fix Issue 22483 - DMD generates invalid string sections that work by coincidence DMD generates bogus ".rodata.str" segment, which isn't really mergeable by proper linkers and this trips https://github.com/rui314/mold linker up. As a workaround, just don't set the SHF_MERGE | SHF_STRINGS flags. https://github.com/dlang/dmd/pull/15915

dlang/dmd pull request #15915 "fix Issue 22483 - DMD generates invalid string sections that work by coincidence" was merged into stable: - 4542057926be4f456d6411059231a2ede7ae33b6 by Siarhei Siamashka: fix Issue 22483 - DMD generates invalid string sections that work by coincidence DMD generates bogus ".rodata.str" segment, which isn't really mergeable by proper linkers and this trips https://github.com/rui314/mold linker up. As a workaround, just don't set the SHF_MERGE | SHF_STRINGS flags. https://github.com/dlang/dmd/pull/15915

dlang/dmd pull request #15978 "merge stable" was merged into master: - 3a85e6bdbee15235d12f714f608a757ad1983bc3 by Siarhei Siamashka: fix Issue 22483 - DMD generates invalid string sections that work by coincidence (#15915) DMD generates bogus ".rodata.str" segment, which isn't really mergeable by proper linkers and this trips https://github.com/rui314/mold linker up. As a workaround, just don't set the SHF_MERGE | SHF_STRINGS flags. https://github.com/dlang/dmd/pull/15978