Link failure when building UML on linux-6.1.y after LLVM commit ec92d74a0ef89

[nathanchance]

After llvm/llvm-project@ec92d74 (according to my bisect), I see several modpost warnings then an error when building UML on linux-6.1.y (ARCH=um has -mcmodel=large as a result of 12783aa):

$ make -skj"$(nproc)" ARCH=um LLVM=1 mrproper defconfig linux
...
WARNING: modpost: vmlinux.o(__ex_table+0x0): Section mismatch in reference from the (unknown reference) (unknown) to the (unknown reference) .ltext:(unknown)
The relocation at __ex_table+0x0 references
section ".ltext" which is not in the list of
authorized sections.  If you're adding a new section
and/or if this reference is valid, add ".ltext" to the
list of authorized sections to jump to on fault.
This can be achieved by adding ".ltext" to
OTHER_TEXT_SECTIONS in scripts/mod/modpost.c.
WARNING: modpost: vmlinux.o(__ex_table+0x4): Section mismatch in reference from the (unknown reference) (unknown) to the (unknown reference) .ltext:(unknown)
The relocation at __ex_table+0x4 references
section ".ltext" which is not in the list of
authorized sections.  If you're adding a new section
and/or if this reference is valid, add ".ltext" to the
list of authorized sections to jump to on fault.
This can be achieved by adding ".ltext" to
OTHER_TEXT_SECTIONS in scripts/mod/modpost.c.
...
/usr/sbin/ld: init/calibrate.o: relocation R_X86_64_32 against `.data' can not be used when making a PIE object; recompile with -fPIE
/usr/sbin/ld: failed to set dynamic section sizes: bad value
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[3]: *** [scripts/Makefile.vmlinux:34: vmlinux] Error 1
...

git bisect log

# bad: [f1156fb622a73bcf1623c9bebfb6a638b097af14] [AMDGPU][IGLP]: Add SchedGroupMask::TRANS (#75416)
# good: [065796bb9293b222fa3d63311af542a98b96d09a] [clang][OpenMP] Fix missing DI for __kmpc_global_thread_num (#73856)
git bisect start 'f1156fb622a73bcf1623c9bebfb6a638b097af14' '065796bb9293b222fa3d63311af542a98b96d09a'
# good: [774295ca1d5ff752cb478b61f22a5b1dbe33074f] [libc++][test] Fix MSVC warnings with `static_cast`s (#74962)
git bisect good 774295ca1d5ff752cb478b61f22a5b1dbe33074f
# good: [2ab926d959613e677458a818e9e42344ab4fd8c2] [flang][MLIR][OpenMP] Add support for `target update` directive. (#75047)
git bisect good 2ab926d959613e677458a818e9e42344ab4fd8c2
# bad: [a3952b4f022ce03c778ecc3b44ffff350b512735] [Analysis] Remove unused forward declarations (NFC)
git bisect bad a3952b4f022ce03c778ecc3b44ffff350b512735
# bad: [c1a6974d6b3dc7be2178d8db9f8fc6c2cc9230f2] [AMDGPU][MC] Add GFX12 SMEM encoding (#75215)
git bisect bad c1a6974d6b3dc7be2178d8db9f8fc6c2cc9230f2
# bad: [3194928c3cfa6623c5de78ae7ab2ab7ab0cf0232] [llvm-exegesis] Refactor MMAP platform-specific preprocessor directives (#75422)
git bisect bad 3194928c3cfa6623c5de78ae7ab2ab7ab0cf0232
# good: [1b531d54f6234488dec048fec1c5aca331bf8f3d] [InstrRef][nfc] Remove usage of unique_ptrs of arrays (#74203)
git bisect good 1b531d54f6234488dec048fec1c5aca331bf8f3d
# good: [5bc1adff69315dcef670e9fcbe04067b5d5963fb] Revert "lldb: Cache string hash during ConstString pool queries/insertions"
git bisect good 5bc1adff69315dcef670e9fcbe04067b5d5963fb
# bad: [0544c781728a665806b069cb8202acd4f6981a0a] Fix a crash from character type confusion interaction with libedit (#75388)
git bisect bad 0544c781728a665806b069cb8202acd4f6981a0a
# bad: [7bc6c4abe8e8d8ab70e02e4c2025a94dda01d908] [mlir][print]Add functions for printing memref f16/bf16/i16 (#75094)
git bisect bad 7bc6c4abe8e8d8ab70e02e4c2025a94dda01d908
# bad: [ec92d74a0ef89b9dd46aee6ec8aca6bfd3c66a54] Reland [X86] Respect code models more when determining if a global reference can fit in 32 bits (#75386)
git bisect bad ec92d74a0ef89b9dd46aee6ec8aca6bfd3c66a54
# good: [f976719fb2cb23364957e5993f7fc3684ee15391] Revert "[ADT][StringMap] Add ability to precompute and reuse the string hash"
git bisect good f976719fb2cb23364957e5993f7fc3684ee15391
# first bad commit: [ec92d74a0ef89b9dd46aee6ec8aca6bfd3c66a54] Reland [X86] Respect code models more when determining if a global reference can fit in 32 bits (#75386)

[nickdesaulniers]

probably need to teach modpost what .ltext is (that it's llvm specific for the large code model) so that it doesn't walk off the end of a list and segfault (I bet). cc @aeubanks

[aeubanks]

.ltext is actually part of the ELF x86-64 psABI. It's basically just .text but also with the SHF_X86_64_LARGE section flag. It's useful for mixing small and large code model object files by distinguishing which functions are "large"/"far" and which ones are "small"/"near". So adding it to the list of allowed section names sgtm.

[nickdesaulniers]

Also, I'm curious if the UM build has been red since that LLVM commit from a few months ago?

[nathanchance]

probably need to teach modpost what .ltext is (that it's llvm specific for the large code model) so that it doesn't walk off the end of a list and segfault (I bet). cc @aeubanks

Will take a look.

Also, I'm curious if the UM build has been red since that LLVM commit from a few months ago?

Yes, I believe it has (at least from my quick survey). I think I missed it initially because of the whole counted_by fiasco that caused our clang-18 to be super red for over a month :/

[nathanchance]

So it turns out -no-pie was not getting added to the link flags because cc-option runs with -c but -no-pie is only used when linking ($(CC) is used for linking, #1715).

$ clang -Werror -no-pie -c -o /dev/null-x c /dev/null
clang-16: error: argument unused during compilation: '-no-pie' [-Werror,-Wunused-command-line-argument]

$ echo 'int main(void) { return 0; }' | clang -Werror -no-pie -o /dev/null -x c -

I think something like this would work, -no-pie is supported with all version of clang and GCC added support for it in 6.1.0 (https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=428b381275bc95032274440f02d7719225de2c17, if I understand correctly):

diff --git a/arch/um/Makefile b/arch/um/Makefile
index 82f05f250634..34957dcb88b9 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -115,7 +115,9 @@ archprepare:
        $(Q)$(MAKE) $(build)=$(HOST_DIR)/um include/generated/user_constants.h

 LINK-$(CONFIG_LD_SCRIPT_STATIC) += -static
-LINK-$(CONFIG_LD_SCRIPT_DYN) += $(call cc-option, -no-pie)
+ifdef CONFIG_LD_SCRIPT_DYN
+LINK-$(call gcc-min-version, 60100)$(CONFIG_CC_IS_CLANG) += -no-pie
+endif
 LINK-$(CONFIG_LD_SCRIPT_DYN_RPATH) += -Wl,-rpath,/lib

 CFLAGS_NO_HARDENING := $(call cc-option, -fno-PIC,) $(call cc-option, -fno-pic,) \

$ boot-uml.py -k linux
...
Linux version 6.7.0-dirty (nathan@distrobox-9ye0OqsCQG.thelio-3990X) (ClangBuiltLinux clang version 18.0.0git (https://github.com/llvm/llvm-project b6f922fbf5e983122271aa12acb33f6172046d4d), ClangBuiltLinux LLD 18.0.0) #1 Wed Jan 17 16:53:21 MST 2024
...

$ boot-uml.py -k linux-stable-6.1
...
Linux version 6.1.73-dirty (nathan@distrobox-9ye0OqsCQG.thelio-3990X) (ClangBuiltLinux clang version 18.0.0git (https://github.com/llvm/llvm-project b6f922fbf5e983122271aa12acb33f6172046d4d), ClangBuiltLinux LLD 18.0.0) #1 Wed Jan 17 16:55:27 MST 2024
...

The warnings are fixed with the diff I have in #1981 (although I am getting a warning when I apply that diff to 6.1...).

I think this has implications for some of the errors David reported in #1715, I will comment there.

[nathanchance]

Patch submitted: https://lore.kernel.org/20240123-fix-uml-clang-18-v1-1-efc095519cf9@kernel.org/

[nathanchance]

Accepted: https://git.kernel.org/masahiroy/linux-kbuild/c/846cfbeed09b45d985079a9173cf390cc053715b

[nathanchance]

Merged into mainline: https://git.kernel.org/linus/846cfbeed09b45d985079a9173cf390cc053715b

This patch has Cc: stable@vger.kernel.org on it, so it will be backported automatically in time.