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

[Luke Nelson]

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>
---
 arch/riscv/net/bpf_jit.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
index 2717f5490428..e159c6e3ff43 100644
--- a/arch/riscv/net/bpf_jit.h
+++ b/arch/riscv/net/bpf_jit.h
@@ -238,7 +238,7 @@ static inline u32 rv_i_insn(u16 imm11_0, u8 rs1, u8 funct3, u8 rd, u8 opcode)

 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;

 	return (imm11_5 << 25) | (rs2 << 20) | (rs1 << 15) | (funct3 << 12) |
 		(imm4_0 << 7) | opcode;
--
2.34.1