mka@ Can you take a look first and check whether this can be fixed in kernel or does it need a clang/linker fix?

My limited understanding is that PLT is used for dynamic linking, which isn't supported in the kernel.

This ancient thread involving a kernel maintainer is very clear about not supporting it:

http://linux-arm-kernel.infradead.narkive.com/QRUkU1L9/r-arm-plt32-relocation

Do we know what changed in LLVM that it wants to use dynamic linking?

Thanks, let me investigate.

arch/x86/realmode/rm/video-vesa.c has a PLT relocation on memset. The issue is present on ToT clang as well.
Doing a bisection to find the CL causing this issue.

Opened https://bugs.llvm.org/show_bug.cgi?id=36674
(The original CL was reverted initially because of a different bug so I should be able to cherry-pick the revert to make the kernel builds work for now)

https://reviews.llvm.org/rL323297
https://reviews.llvm.org/rL325514

A comment on the LLVM bug (https://bugs.llvm.org/show_bug.cgi?id=36674#c3) mentions a kernel patch, which actually has already been merged to the upstream kernel:

commit b21ebf2fb4cde1618915a97cc773e287ff49173e
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Feb 7 14:20:09 2018 -0800

    x86: Treat R_X86_64_PLT32 as R_X86_64_PC32

We can cherry pick this patch to v4.4 and v4.14 instead of reverting the LLVM change.


This kind of change still sucks, since it breaks builds of 'older' kernels (including v4.14 LTS) not only for us but for everybody using a ToTish LLVM :/

Thanks Matthias, I was just going to ask you.
Maybe you (or ask HJL) can cherry-pick it to 4.4/4.14 kernel LTS trees ?

Support for R_386_PLT32 is still missing.

mka@ can you send this patch Linux kernel upstream?

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 005c03e93fc5..b88bfaf4e991 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -124,6 +124,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
 			*location += sym->st_value;
 			break;
 		case R_386_PC32:
+		case R_386_PLT32:
 			/* Add the value, subtract its position */
 			*location += sym->st_value - (uint32_t)location;
 			break;
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 73eb7fd4aec4..fb4382a7f949 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -834,6 +835,7 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
 	switch (r_type) {
 	case R_386_NONE:
 	case R_386_PC32:
+	case R_386_PLT32:
 	case R_386_PC16:
 	case R_386_PC8:
 		/*
@@ -877,6 +879,7 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
 	switch (r_type) {
 	case R_386_NONE:
 	case R_386_PC32:
+	case R_386_PLT32:
 	case R_386_PC16:
 	case R_386_PC8:
 		/*

At first sight a port of the x86_64 patch to i386 seems trivial, however I am not sure if that trivial port is correct.

From the commit message of the x86_64 patch:

"On i386, there are 2 types of PLTs, PIC and non-PIC.  PIE and shared
objects must use PIC PLT.  To use PIC PLT, you need to load
_GLOBAL_OFFSET_TABLE_ into EBX first.  There is no need for that on
x86-64 since x86-64 uses PC-relative PLT."

manoj, could you reply to the LLVM bug (where the author of the patch posted) that the kernel patch doesn't fix the reported problem with R_386_PLT32 and try to get some more information on what would be the correct solution?

HJL just replied on the bug. So I hope Rafael will take care of fixing it.
Meanwhile, I can fix our builds by cherry-picking the revert if the fix takes some time.

Rafael send he fix for R_386_PLT32 at https://reviews.llvm.org/D44334

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/20d214bc1b568d202de358041734f83ab3eb6bf0

commit 20d214bc1b568d202de358041734f83ab3eb6bf0
Author: Manoj Gupta <manojgupta@google.com>
Date: Sat Mar 10 06:59:17 2018

llvm-next: Cherry-pick 4 upstream CLs.

Cherry-pick 4 CLs to fix various issues. These are cherry-picked to
Chrome OS's llvm-next only. Does not impact the current llvm
version used.

CL descriptions:

commit 284236c047631c8b0eabac3ddd3d0c95253f4361
Author: Manoj Gupta <manojgupta@google.com>
Date:   Fri Mar 2 23:52:44 2018 +0000

    Do not generate calls to fentry with __attribute__((no_instrument_function))

commit 30f9051d7d8b6f56c8149fd1bdcc714285f77527
Author: Dan Albert <danalbert@google.com>
Date:   Wed Feb 21 22:36:51 2018 +0000

    [Driver] Generate .eh_frame_hdr for static executables too.

commit 98079e294f718c14d25ccf30ab2b1938780ffe4d
Author: Florian Hahn <florian.hahn@arm.com>
Date:   Fri Mar 2 13:02:55 2018 +0000

    [ARM] Fix codegen for VLD3/VLD4/VST3/VST4 with WB

commit ffaaef68dbab0e872c0e6013836170bb78705a81
Author: Reid Kleckner <rnk@google.com>
Date:   Tue Feb 6 00:47:14 2018 +0000

    Revert "Don't assume a null GV is local for ELF and MachO."

BUG= chromium:820140 
BUG= chromium:815357 
BUG=b:74090329

TEST="USE=llvm-next" sudo emerge  llvm works.

Change-Id: I67d2215dd47dc9ada2381b8817b485c9f8a0d107
Reviewed-on: https://chromium-review.googlesource.com/957474
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>

[add] https://crrev.com/20d214bc1b568d202de358041734f83ab3eb6bf0/sys-devel/llvm/files/cherry/30f9051d7d8b6f56c8149fd1bdcc714285f77527.patch
[add] https://crrev.com/20d214bc1b568d202de358041734f83ab3eb6bf0/sys-devel/llvm/files/cherry/98079e294f718c14d25ccf30ab2b1938780ffe4d.patch
[add] https://crrev.com/20d214bc1b568d202de358041734f83ab3eb6bf0/sys-devel/llvm/files/cherry/284236c047631c8b0eabac3ddd3d0c95253f4361.patch
[rename] https://crrev.com/20d214bc1b568d202de358041734f83ab3eb6bf0/sys-devel/llvm/llvm-6.0_pre321490_p20180131-r7.ebuild
[add] https://crrev.com/20d214bc1b568d202de358041734f83ab3eb6bf0/sys-devel/llvm/files/cherry/ffaaef68dbab0e872c0e6013836170bb78705a81.patch

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/655a49d1a7385c14f9822a39673970a05b41a00c

commit 655a49d1a7385c14f9822a39673970a05b41a00c
Author: Manoj Gupta <manojgupta@google.com>
Date: Tue Mar 13 04:03:06 2018

llvm-next Update to r326829.

Update llvm-next to current crosstool version.
In addition, add a few cherry picks to fix issues
related to Linxu kernel:
1. PLT relocations: r327198.
2. Increased stack usage: r327192 and r327229.

Note: Does not affect the current llvm used in Chrome OS.

BUG= chromium:817628 
BUG= chromium:820140 
BUG=chromium:815537

TEST=USE="llvm-next" sudo emerge llvm lld works.

Change-Id: I436d578030775b3ffbccb246dbd43394452e67a3
Reviewed-on: https://chromium-review.googlesource.com/957790
Trybot-Ready: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Luis Lozano <llozano@chromium.org>

[add] https://crrev.com/655a49d1a7385c14f9822a39673970a05b41a00c/sys-devel/llvm/files/cherry/8fdc88794b44e70bdb93c6cf04baf3c1e3251d8b.patch
[delete] https://crrev.com/bd99bff64464733038e8b5cdc488fd9f4941aea6/sys-devel/llvm/files/cherry/98079e294f718c14d25ccf30ab2b1938780ffe4d.patch
[rename] https://crrev.com/655a49d1a7385c14f9822a39673970a05b41a00c/sys-devel/lld/lld-6.0_pre321490-r3.ebuild
[delete] https://crrev.com/bd99bff64464733038e8b5cdc488fd9f4941aea6/sys-devel/llvm/files/cherry/30f9051d7d8b6f56c8149fd1bdcc714285f77527.patch
[delete] https://crrev.com/bd99bff64464733038e8b5cdc488fd9f4941aea6/sys-devel/llvm/files/cherry/16748767563bb9bcb1e1c3e42c35d44924d464d0.patch
[rename] https://crrev.com/655a49d1a7385c14f9822a39673970a05b41a00c/sys-devel/llvm/llvm-6.0_pre321490_p20180131-r9.ebuild
[add] https://crrev.com/655a49d1a7385c14f9822a39673970a05b41a00c/sys-devel/llvm/files/cherry/824eedb9eb4888575924b1ed80c4250dddd5b59b.patch
[delete] https://crrev.com/bd99bff64464733038e8b5cdc488fd9f4941aea6/sys-devel/llvm/files/cherry/284236c047631c8b0eabac3ddd3d0c95253f4361.patch
[rename] https://crrev.com/655a49d1a7385c14f9822a39673970a05b41a00c/sys-libs/compiler-rt/compiler-rt-6.0_pre321490-r5.ebuild
[delete] https://crrev.com/bd99bff64464733038e8b5cdc488fd9f4941aea6/sys-devel/llvm/files/cherry/ffaaef68dbab0e872c0e6013836170bb78705a81.patch
[add] https://crrev.com/655a49d1a7385c14f9822a39673970a05b41a00c/sys-devel/llvm/files/cherry/c28eb6d02c5cedd40b02aa7c496f13a71763312e.patch