Re: [PATCH bpf-next] selftests/bpf: Fix some incorrect inline asm codes

[Yonghong Song <yonghong.song@linux.dev>]

On 6/12/25 12:10 PM, Eduard Zingerman wrote:
> On Thu, 2025-06-12 at 10:19 -0700, Yonghong Song wrote:
>> In one of upstream thread ([1]), there is a discussion about
>> the below inline asm code:
>>
>>    if r1 == 0xdeadbeef goto +2;
>>    ...
>>
>> In actual llvm backend, the above 0xdeadbeef will actually do
>> sign extension to 64bit value and then compare to register r1.
>>
>> But the code itself does not imply the above semantics. It looks
>> like the comparision is between r1 and 0xdeadbeef. For example,
>> let us at a simple C code:
>>    $ cat t1.c
>>    int foo(long a) { return a == 0xdeadbeef ? 2 : 3; }
>>    $ clang --target=bpf -O2 -c t1.c && llvm-objdump -d t1.o
>>      ...
>>      w0 = 0x2
>>      r2 = 0xdeadbeef ll
>>      if r1 == r2 goto +0x1
>>      w0 = 0x3
>>      exit
>> It does try to compare r1 and 0xdeadbeef.
>>
>> To address the above confusing inline asm issue, llvm backend ([2])
>> added some range checking for such insns and beyond. For the above
>> insn asm, the warning like below
>>    warning: immediate out of range, shall fit in int range
>> will be issued. If -Werror is in the compilation flags, the
>> error will be issued.
>>
>> To avoid the above warning/error, the afore-mentioned inline asm
>> should be rewritten to
>>
>>    if r1 == -559038737 goto +2;
>>    ...
>>
>> Fix a few selftest cases like the above based on insn range checking
>> requirement in [2].
>>
>>    [1] https://lore.kernel.org/bpf/70affb12-327b-4882-bd1d-afda8b8c6f56@linux.dev/
>>    [2] https://github.com/llvm/llvm-project/pull/142989
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
> Changes like 0xffffffff -> -1 and 0xfffffffe -> -2 look fine,
> but changes like 0xffff1234 -> -60876 are an unnecessary obfuscation,
> maybe we need to reconsider.

Another option is to generate below in inline asm:
     r2 = 0xffffFFFFffff1234  /* or r2 = 0xffff1234, depending on the actual user expectation */
     if (r1 == r2) ...

>
> [...]