On Tue, Jun 15, 2021 at 02:32:18PM -0700, Eric Biggers wrote: > On Tue, Jun 15, 2021 at 11:08:18PM +0200, Daniel Borkmann wrote: > > On 6/15/21 9:33 PM, Eric Biggers wrote: > > > On Tue, Jun 15, 2021 at 07:51:07PM +0100, Edward Cree wrote: > > > > > > > > As I understand it, the UBSAN report is coming from the eBPF interpreter, > > > > which is the *slow path* and indeed on many production systems is > > > > compiled out for hardening reasons (CONFIG_BPF_JIT_ALWAYS_ON). > > > > Perhaps a better approach to the fix would be to change the interpreter > > > > to compute "DST = DST << (SRC & 63);" (and similar for other shifts and > > > > bitnesses), thus matching the behaviour of most chips' shift opcodes. > > > > This would shut up UBSAN, without affecting JIT code generation. > > > > > > Yes, I suggested that last week > > > (https://lkml.kernel.org/netdev/YMJvbGEz0xu9JU9D@xxxxxxxxx). The AND will even > > > get optimized out when compiling for most CPUs. > > > > Did you check if the generated interpreter code for e.g. x86 is the same > > before/after with that? > > Yes, on x86_64 with gcc 10.2.1, the disassembly of ___bpf_prog_run() is the same > both before and after (with UBSAN disabled). Here is the patch I used: > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > index 5e31ee9f7512..996db8a1bbfb 100644 > --- a/kernel/bpf/core.c > +++ b/kernel/bpf/core.c > @@ -1407,12 +1407,30 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn) > DST = (u32) DST OP (u32) IMM; \ > CONT; > > + /* > + * Explicitly mask the shift amounts with 63 or 31 to avoid undefined > + * behavior. Normally this won't affect the generated code. > + */ > +#define ALU_SHIFT(OPCODE, OP) \ > + ALU64_##OPCODE##_X: \ > + DST = DST OP (SRC & 63);\ > + CONT; \ > + ALU_##OPCODE##_X: \ > + DST = (u32) DST OP ((u32)SRC & 31); \ > + CONT; \ > + ALU64_##OPCODE##_K: \ > + DST = DST OP (IMM & 63); \ > + CONT; \ > + ALU_##OPCODE##_K: \ > + DST = (u32) DST OP ((u32)IMM & 31); \ > + CONT; > + > ALU(ADD, +) > ALU(SUB, -) > ALU(AND, &) > ALU(OR, |) > - ALU(LSH, <<) > - ALU(RSH, >>) > + ALU_SHIFT(LSH, <<) > + ALU_SHIFT(RSH, >>) > ALU(XOR, ^) > ALU(MUL, *) > #undef ALU > Note, I missed the arithmetic right shifts later on in the function. Same result there, though. - Eric
