Date: Mon, 15 Oct 2018 15:44:40 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 232291] ld.bfd (newer) and ld.lld (6 and imho 7) create empty sections when they should not Message-ID: <bug-232291-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D232291 Bug ID: 232291 Summary: ld.bfd (newer) and ld.lld (6 and imho 7) create empty sections when they should not Product: Base System Version: CURRENT Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: bin Assignee: bugs@FreeBSD.org Reporter: bz@FreeBSD.org CC: arichardson@FreeBSD.org, dim@FreeBSD.org, emaste@freebsd.org Working on a possible solution for PR230857 it seemed that ld.bfd and ld.lld are bug compatible. The old version of ld.bfd currently in HEAD (GNU ld 2.17.50 [FreeBSD] 2007-07-03 , from before the branch of stable/12) seems to do the expected thing on certain statements with regards to "Dot". Newer versions of both ld.bfd (2.26) and ld.lld seem to fail to do the right thing. Strangely enough in old times that behaviour was not documented and worked,= for newer version is documented and fails. One of the possible solution for the above mentioned PR as initially outlin= ed in https://reviews.freebsd.org/D17512 was along the lines of the following linker script snippet: 39 SECTIONS 40 { 41 set_pcpu : 42 { 43 *(set_pcpu) 44 . =3D . + ABSOLUTE (___set_pcpu_pad) ; 45 } 46 } If ___set_pcpu_pad would be 0 the statement would be . =3D . + (sym=3D=3D0)= in which case, was the section non-existent/empty before should not be created. How= ever it is with lld and the above mentioned bfd version at least. According to https://sourceware.org/binutils/docs-2.31/ld/Output-Section-Discarding.html= #Output-Section-Discarding it should not be created: QUOTE >>> Other link script directives that allocate space in an output sec= tion will also create the output section. So too will assignments to dot even if= the assignment does not create space, except for =E2=80=98. =3D 0=E2=80=99, =E2= =80=98. =3D . + 0=E2=80=99, =E2=80=98. =3D sym=E2=80=99, =E2=80=98. =3D . + sym=E2=80=99 and =E2=80=98. =3D ALIGN (. !=3D 0, expr, 1= )=E2=80=99 when =E2=80=98sym=E2=80=99 is an absolute symbol of value 0 defined in the script. This allows you to force output of= an empty section with =E2=80=98. =3D .=E2=80=99. <<< We should make sure that the upstream linkers we use get fixed to properly = work as documented and as at least one seems to have in the past. Having bug-compatiility between the two is not helpful at all. While they are broken we might have to end up splattering sections over all kernel modules, adding BYTE(1)-kind padding to these sections and checking = this and filtering this out in the in-kernel linker, which is extra noise, extra size, extra complexity, .. otherwise not needed, as well as possible proble= ms with out-of-tree compiled kernel modules. --=20 You are receiving this mail because: You are the assignee for the bug.=
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