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:

quoted

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)

quoted

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".

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?

How do you think? If you think it is the solution, I'll do that work.