On Sat, May 15, 2021 at 11:24:25PM +0200, Vlastimil Babka wrote:
On 5/15/21 11:09 PM, Hyeonggon Yoo wrote:
Hello Vlastimil, recently kbuild-all test bot reported compile error on clang 10.0.1, with defconfig.
Hm yes, catching some compiler bug was something that was noted to be possible to happen.
Nathan Chancellor wrote:
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. Cheers, Nathan
I think it's because kmalloc compiles successfully when size is constant, and kmalloc_index isn't. so I think compiler seems to be confused.
currently if size is non-constant, kmalloc calls dummy function __kmalloc, which always returns NULL.
That's a misunderstanding. __kmalloc() is not a dummy function, you probably found only the header declaration.
so what about changing kmalloc to do compile-time assertion too, and track all callers that are calling kmalloc with non-constant argument.
kmalloc() is expected to be called with both constant and non-constant size. __builtin_constant_p() is used to determine which implementation to use. One based on kmalloc_index(), other on __kmalloc().
It appears clang 10.0.1 is mistakenly evaluating __builtin_constant_p() as true. Probably something to do with LTO, because MAX_OPTINSN_SIZE seems it could be a "link-time constant".
This happens with x86_64 defconfig so LTO is not involved.
However, the explanation makes sense, given that the LLVM change I landed on changes the sparse conditional constant propagation pass, which I believe can influence how LLVM handles __builtin_constant_p().
Maybe we could extend Marco Elver's followup patch that uses BUILD_BUG_ON vs BUG() depending on size_is_constant parameter. It could use BUG() also if the compiler is LLVM < 11 or something. What would be the proper code for this condition?
This should work I think:
diff --git a/include/linux/slab.h b/include/linux/slab.h index 9d316aac0aba..1b653266f2aa 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -413,7 +413,7 @@ static __always_inline unsigned int __kmalloc_index(size_t size, if (size <= 16 * 1024 * 1024) return 24; if (size <= 32 * 1024 * 1024) return 25;
- if (size_is_constant) + if ((IS_ENABLED(CONFIG_CC_IS_GCC) || CONFIG_CLANG_VERSION > 110000) && size_is_constant) BUILD_BUG_ON_MSG(1, "unexpected size in kmalloc_index()"); else BUG();