On Fri, May 14, 2021 at 1:54 PM Nathan Chancellor [off-list ref] wrote:

On 5/14/2021 11:12 AM, Naresh Kamboju wrote:

quoted

My two cents,
We know clang-10 support is stopped, but our build system is still running

For the record, we are still building with clang-10 as that is the
minimum supported version for the kernel.

quoted

these builds with clang-10. one of those observations is,

The LKFT build failures were noticed while building x86_64 and i386 with
clang-10 on linux next-20210514 tag.

   - x86_64 (defconfig) with clang-10 - FAIL
   - i386 (defconfig) with clang-10 - FAIL
   - x86_64 (defconfig) with clang-11 - PASS
   - i386 (defconfig) with clang-11 - PASS
   - x86_64 (defconfig) with clang-12 - PASS
   - i386 (defconfig) with clang-12 - PASS

Build log:
----------
make --silent --keep-going --jobs=8
O=/home/tuxbuild/.cache/tuxmake/builds/current ARCH=x86_64
CROSS_COMPILE=x86_64-linux-gnu- 'HOSTCC=sccache clang' 'CC=sccache
clang'

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:
eb_relocate_parse_slow()+0x427: stack state mismatch: cfa1=4+120
cfa2=-1+0
drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:
eb_copy_relocations()+0x1d5: stack state mismatch: cfa1=4+104
cfa2=-1+0
x86_64-linux-gnu-ld: arch/x86/kernel/kprobes/opt.o: in function
`arch_prepare_optimized_kprobe':
opt.c:(.text+0x798): undefined reference to `__compiletime_assert_251'
make[1]: *** [/builds/linux/Makefile:1272: vmlinux] Error 1

Thanks for the report. Our CI also reported this:

https://github.com/ClangBuiltLinux/continuous-integration2/runs/2582534773?check_suite_focus=true

My bisect landed on "mm, slub: change run-time assertion in
kmalloc_index() to compile-time" in -next (I have added the author and
reviewers to cc):

$ git bisect log
# bad: [cd557f1c605fc5a2c0eb0b540610f50dc67dd849] Add linux-next
specific files for 20210514
# good: [315d99318179b9cd5077ccc9f7f26a164c9fa998] Merge tag
'pm-5.13-rc2' of
git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm
git bisect start 'cd557f1c605fc5a2c0eb0b540610f50dc67dd849'
'315d99318179b9cd5077ccc9f7f26a164c9fa998'
# good: [9634d7cb3c506ae886a5136d12b4af696b9cee8a] Merge remote-tracking
branch 'drm-misc/for-linux-next'
git bisect good 9634d7cb3c506ae886a5136d12b4af696b9cee8a
# good: [294636a24ae819a7caf0807d05d8eb5b964ec06f] Merge remote-tracking
branch 'rcu/rcu/next'
git bisect good 294636a24ae819a7caf0807d05d8eb5b964ec06f
# good: [cb753d0611f912439c8e814f4254d15fa8fa1d75] Merge remote-tracking
branch 'gpio-brgl/gpio/for-next'
git bisect good cb753d0611f912439c8e814f4254d15fa8fa1d75
# bad: [b1e7389449084b74a044a70860c6a1c7466781cb] lib/string_helpers:
switch to use BIT() macro
git bisect bad b1e7389449084b74a044a70860c6a1c7466781cb
# bad: [bf5570ed0654a21000e5dad9243ea1ba30bfe208] kasan: use
dump_stack_lvl(KERN_ERR) to print stacks
git bisect bad bf5570ed0654a21000e5dad9243ea1ba30bfe208
# bad: [4a292ff7a819404039588c7a9af272aca22c869e] fixup! mm: gup: pack
has_pinned in MMF_HAS_PINNED
git bisect bad 4a292ff7a819404039588c7a9af272aca22c869e
# good: [93e440dd0e482b6cde20630d58016a451fb2b431] ocfs2: fix snprintf()
checking
git bisect good 93e440dd0e482b6cde20630d58016a451fb2b431
# good: [f32aeffdb407b6390ef17ccac87e1a1a76663354] slub: remove
resiliency_test() function
git bisect good f32aeffdb407b6390ef17ccac87e1a1a76663354
# bad: [2c9b936dae88dbdbb28666a741c87b08abb1b755] mm/page-writeback:
update the comment of Dirty position control
git bisect bad 2c9b936dae88dbdbb28666a741c87b08abb1b755
# bad: [597da4750c8e8f5f304419f3c66b713a88f3ebbe]
tools/vm/page_owner_sort.c: check malloc() return
git bisect bad 597da4750c8e8f5f304419f3c66b713a88f3ebbe
# bad: [0ec3603432be3abba93ee7aa512cb6d83dd1c291] kfence: test: fix for
"mm, slub: change run-time assertion in kmalloc_index() to compile-time"
git bisect bad 0ec3603432be3abba93ee7aa512cb6d83dd1c291
# bad: [ff3daafe3fd3e54c8cf6aeb865099c7bb02e5c51] mm, slub: change
run-time assertion in kmalloc_index() to compile-time
git bisect bad ff3daafe3fd3e54c8cf6aeb865099c7bb02e5c51
# first bad commit: [ff3daafe3fd3e54c8cf6aeb865099c7bb02e5c51] mm, slub:
change run-time assertion in kmalloc_index() to compile-time

https://git.kernel.org/next/linux-next/c/ff3daafe3fd3e54c8cf6aeb865099c7bb02e5c51

It looks like LLVM fixed this in
http://github.com/llvm/llvm-project/commit/6aabb109becfee37126ff7db3d41a41b5779f359.

I think this happens because arch_prepare_optimized_kprobe() calls
kzalloc() with a size of MAX_OPTINSN_SIZE, which is

#define MAX_OPTINSN_SIZE                                \
        (((unsigned long)optprobe_template_end -        \
          (unsigned long)optprobe_template_entry) +     \
         MAX_OPTIMIZED_LENGTH + JMP32_INSN_SIZE)

and the optprobe_template_{end,entry} are not evaluated as constants.

I am not sure what the solution is. There seem to be a growing list of
issues with LLVM 10 that were fixed in LLVM 11, which might necessitate
requiring LLVM 11 and newer to build the kernel, given this affects a
defconfig.

If we can make the compile time validation done instead at runtime for
clang-10, that would be preferable. Then when we drop clang-10
support, we can revert that.  Looking at the diff, I think we could
reinstate parts like so:

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 9d316aac0aba..4c13e2cd415c 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h

@@ -412,6 +412,9 @@ static __always_inline unsigned int

__kmalloc_index(size_t size,
        if (size <=  8 * 1024 * 1024) return 23;
        if (size <=  16 * 1024 * 1024) return 24;
        if (size <=  32 * 1024 * 1024) return 25;
+#if CLANG_VERSION <= 100000
+       if (size <=  64 * 1024 * 1024) return 26;
+#endif

        if (size_is_constant)
                BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()");

diff --git a/mm/slab_common.c b/mm/slab_common.c
index a0b8ffeae4ae..ecb6104b9dbb 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c

@@ -799,6 +799,9 @@ const struct kmalloc_info_struct kmalloc_info[]

__initconst = {
        INIT_KMALLOC_INFO(8388608, 8M),
        INIT_KMALLOC_INFO(16777216, 16M),
        INIT_KMALLOC_INFO(33554432, 32M)
+#if CLANG_VERSION <= 10000
+       ,INIT_KMALLOC_INFO(67108864, 64M)
+#endif
 };

 /*
-- 
Thanks,
~Nick Desaulniers