On 2021/10/27 19:50, Mark Rutland wrote:
On Wed, Oct 27, 2021 at 11:18:22AM +0000, Tong Tiangen wrote:When a tracing BPF program attempts to read memory without using the bpf_probe_read() helper, the verifier marks the load instruction with the BPF_PROBE_MEM flag. Since the riscv JIT does not currently recognize this flag it falls back to the interpreter. Add support for BPF_PROBE_MEM, by appending an exception table to the BPF program. If the load instruction causes a data abort, the fixup infrastructure finds the exception table and fixes up the fault, by clearing the destination register and jumping over the faulting instruction. A more generic solution would add a "handler" field to the table entry, like on x86 and s390. The same issue in ARM64 is fixed in: commit 800834285361 ("bpf, arm64: Add BPF exception tables")+#ifdef CONFIG_BPF_JIT +int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs); +#endif + int fixup_exception(struct pt_regs *regs) { const struct exception_table_entry *fixup; fixup = search_exception_tables(regs->epc); - if (fixup) { - regs->epc = fixup->fixup; - return 1; - } - return 0; + if (!fixup) + return 0; + +#ifdef CONFIG_BPF_JIT + if (regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END) + return rv_bpf_fixup_exception(fixup, regs); +#endif + + regs->epc = fixup->fixup; + return 1; }As a heads-up, on the extable front, both arm64 and x86 are moving to having an enumerated "type" field to select the handler: x86: https://lore.kernel.org/lkml/20210908132525.211958725@xxxxxxxxxxxxx/ arm64: https://lore.kernel.org/linux-arm-kernel/20211019160219.5202-11-mark.rutland@xxxxxxx/ ... and going forwards, riscv might want to do likewise. Thanks, Mark. .
Thanks mark, I'm very interested in this change.
