On Tue, Jul 28, 2020 at 05:21:26PM +0200, Jean-Philippe Brucker 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 arm64 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. > > To keep the compact exception table entry format, inspect the pc in > fixup_exception(). A more generic solution would add a "handler" field > to the table entry, like on x86 and s390. > > Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx> This will fail to compile on arm64, https://gitlab.com/cailca/linux-mm/-/blob/master/arm64.config arch/arm64/mm/extable.o: In function `fixup_exception': arch/arm64/mm/extable.c:19: undefined reference to `arm64_bpf_fixup_exception' > --- > Note: the extable is aligned on 32 bits. Given that extable entries have > 32-bit members I figured we don't need to align it to 64 bits. > --- > arch/arm64/include/asm/extable.h | 3 ++ > arch/arm64/mm/extable.c | 11 ++-- > arch/arm64/net/bpf_jit_comp.c | 93 +++++++++++++++++++++++++++++--- > 3 files changed, 98 insertions(+), 9 deletions(-) > > diff --git a/arch/arm64/include/asm/extable.h b/arch/arm64/include/asm/extable.h > index 56a4f68b262e..bcee40df1586 100644 > --- a/arch/arm64/include/asm/extable.h > +++ b/arch/arm64/include/asm/extable.h > @@ -22,5 +22,8 @@ struct exception_table_entry > > #define ARCH_HAS_RELATIVE_EXTABLE > > +int arm64_bpf_fixup_exception(const struct exception_table_entry *ex, > + struct pt_regs *regs); > + > extern int fixup_exception(struct pt_regs *regs); > #endif > diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c > index 81e694af5f8c..1f42991cacdd 100644 > --- a/arch/arm64/mm/extable.c > +++ b/arch/arm64/mm/extable.c > @@ -11,8 +11,13 @@ int fixup_exception(struct pt_regs *regs) > const struct exception_table_entry *fixup; > > fixup = search_exception_tables(instruction_pointer(regs)); > - if (fixup) > - regs->pc = (unsigned long)&fixup->fixup + fixup->fixup; > + if (!fixup) > + return 0; > > - return fixup != NULL; > + if (regs->pc >= BPF_JIT_REGION_START && > + regs->pc < BPF_JIT_REGION_END) > + return arm64_bpf_fixup_exception(fixup, regs); > + > + regs->pc = (unsigned long)&fixup->fixup + fixup->fixup; > + return 1; > } > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c > index 3cb25b43b368..f8912e45be7a 100644 > --- a/arch/arm64/net/bpf_jit_comp.c > +++ b/arch/arm64/net/bpf_jit_comp.c > @@ -7,6 +7,7 @@ > > #define pr_fmt(fmt) "bpf_jit: " fmt > > +#include <linux/bitfield.h> > #include <linux/bpf.h> > #include <linux/filter.h> > #include <linux/printk.h> > @@ -56,6 +57,7 @@ struct jit_ctx { > int idx; > int epilogue_offset; > int *offset; > + int exentry_idx; > __le32 *image; > u32 stack_size; > }; > @@ -351,6 +353,67 @@ static void build_epilogue(struct jit_ctx *ctx) > emit(A64_RET(A64_LR), ctx); > } > > +#define BPF_FIXUP_OFFSET_MASK GENMASK(26, 0) > +#define BPF_FIXUP_REG_MASK GENMASK(31, 27) > + > +int arm64_bpf_fixup_exception(const struct exception_table_entry *ex, > + struct pt_regs *regs) > +{ > + off_t offset = FIELD_GET(BPF_FIXUP_OFFSET_MASK, ex->fixup); > + int dst_reg = FIELD_GET(BPF_FIXUP_REG_MASK, ex->fixup); > + > + regs->regs[dst_reg] = 0; > + regs->pc = (unsigned long)&ex->fixup - offset; > + return 1; > +} > + []
