[PATCH bpf-next] riscv/bpf: Fix truncated immediate warning in rv_s_insn

[Luke Nelson]

>>  static inline u32 rv_s_insn(u16 imm11_0, u8 rs2, u8 rs1, u8 funct3, u8 opcode)
>>  {
>> - u8 imm11_5 = imm11_0 >> 5, imm4_0 = imm11_0 & 0x1f;
>> + u32 imm11_5 = (imm11_0 >> 5) & 0x7f, imm4_0 = imm11_0 & 0x1f;
> 
> Hi Luke,
> 
> keep u8 and add 0x7f explicit mask should work. I ran the repro case and it can silence the warning.
> 
>> 
>>   return (imm11_5 << 25) | (rs2 << 20) | (rs1 << 15) | (funct3 << 12) |
>>   (imm4_0 << 7) | opcode;

That does fix the warning, but I think explicitly declaring imm11_5
as u32 is more clear here than the current code which relies on
implicit promotion of imm11_5 from u8 to signed int in the expression
(imm11_5 << 25).

Because of the promotion to signed int, (imm11_5 << 25) is technically
signed overflow and undefined behavior whenever the shift changes
the value in the sign bit. In practice, this isn't an issue; both
because the kernel is compiled with -fno-strict-overflow, but also
because GCC documentation explicitly states that "GCC does not use
the latitude given in C99 and C11 only to treat certain aspects of
signed '<<' as undefined" [1].

Though it may not be an issue in practice, since I'm touching this
line anyways to fix the warning, I think it makes sense to update
the type of imm11_5 to be u32 at the same time.

> Nit: maybe use "riscv, bpf" for the subject will look nice for the riscv-bpf git log tree.

Sure, I can send out a new revision with an updated subject line.

- Luke


[1]: https://gcc.gnu.org/onlinedocs/gcc/Integers-implementation.html