Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: [...] > > What is the expected value for the following 64-bit BPF_DIV operation: > > r0 = 0xFFFFFFFFFFFFFFFF > > r0 /= -10 > > Is it 0x1 or 0x10000000a? i.e., is the -10 sign extended to > > 0xFFFFFFFFFFFFFFF6 or treated as 0xFFFFFFF6 when doing the unsigned > > division? > > x86 and arm64 JITs treat it as imm32 is zero extended. Alan Jowett just pointed out to me that the question is not limited to DIV. r0 = 1 r0 += -1 Is the answer 0 or 0x0000000100000000? Assuming the answer is to zero extend imm32, it contains the latter, which would be counter-intuitive enough to make it important to point out explicitly. > But looking at the interpreter: > ALU64_DIV_K: > DST = div64_u64(DST, IMM); it looks like we have a bug there. > But we have a bunch of div_k tests in lib/test_bpf.c including negative > imm32. Hmm. Yeah. "ALU64_DIV_K: 0xffffffffffffffff / (-1) = 0x0000000000000001", "ALU64_ADD_K: 2147483646 + -2147483647 = -1", "ALU64_ADD_K: 0 + (-1) = 0xffffffffffffffff", "ALU64_MUL_K: 1 * -2147483647 = -2147483647", "ALU64_MUL_K: 1 * (-1) = 0xffffffffffffffff", "ALU64_AND_K: 0x0000ffffffff0000 & -1 = 0x0000ffffffff0000", "ALU64_AND_K: 0xffffffffffffffff & -1 = 0xffffffffffffffff", "ALU64_OR_K: 0x000000000000000 | -1 = 0xffffffffffffffff", The above assume sign extension not zero extension is the correct behavior for these operations, if I understand correctly. Dave
