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

[Björn Töpel]

Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx> writes:

> Sparse warns that a cast in rv_s_insn truncates bits from the constant
> 0x7ff to 0xff.  The warning originates from the use of a constant offset
> of -8 in a store instruction in bpf_jit_comp64.c:
>
>   emit(rv_sd(RV_REG_SP, -8, RV_REG_RA), &ctx);
>
> rv_sd then calls rv_s_insn, with imm11_0 equal to (u16)(-8), or 0xfff8.
>
> Here's the current implementation of rv_s_insn:
>
>   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;
>
>           return (imm11_5 << 25) | (rs2 << 20) | (rs1 << 15) | (funct3 << 12) |
>                  (imm4_0 << 7) | opcode;
>   }
>
> imm11_0 is a signed 12-bit immediate offset of the store instruction. The
> instruction encoding requires splitting the immediate into bits 11:5 and
> bits 4:0. In this case, imm11_0 >> 5 = 0x7ff, which then gets truncated
> to 0xff when cast to u8, causing the warning from sparse. However, this is
> not actually an issue because the immediate offset is signed---truncating
> upper bits that are all set to 1 has no effect on the value of the
> immediate.
>
> There is another subtle quirk with this code, which is imm11_5 is
> supposed to be the upper 7 bits of the 12-bit signed immediate, but its
> type is u8 with no explicit mask to select out only the bottom 7 bits.
> This happens to be okay here because imm11_5 is the left-most field in
> the instruction and the "extra" bit will be shifted out when imm11_5 is
> shifted left by 25.
>
> This commit fixes the warning by changing the type of imm11_5 and imm4_0
> to be u32 instead of u8, and adding an explicit mask to compute imm11_5
> instead of relying on truncation + shifting.
>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202307260704.dUElCrWU-lkp@xxxxxxxxx/
> In-Reply-To: <202307260704.dUElCrWU-lkp@xxxxxxxxx>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>
> Cc: Xi Wang <xi.wang@xxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>


Thanks, Luke!

Acked-by: Björn Töpel <bjorn@xxxxxxxxxx>