[RFC PATCH -next v2 0/4] arm64/ftrace: support dynamic trampoline

Wang ShaoBo
2022-03-16T09:53:32Z

This implements dynamic trampoline in ARM64, as reference said, we
complete whole design of supporting long jump in dynamic trampoline:

.text section:
funcA: | funcA(): funcB():|
`-> +-----+ | | ... mov x9 |
| ... | | | adrp <- bl <> |
| nop | | | mov
| nop | | | br x16 ---+
funcB | nop | | | ftrace_(regs_)caller_tramp:
`-> +-----+ | `--> +---------------------+
| nop | | | ... |
| nop | | ftrace callsite +---------------------+
| ... | | `-----> | PLT entry: |
| nop | | | adrp |
| nop | | | add |
funcC: | nop | | ftrace graph callsite | br x16 |
`-> +-----+ | `-----> +---------------------+
| nop | | | ... |
| nop | | +---------------------+

But there is still a tricky problem that is how to adjust tracing ip,
waiting to be solved:

For ARM64, somecases there may be extra instructions inserted into the
head of tracable functions(but not all) by compiler, for instance BTI[1].

This dump vmlinux with CONFIG_BTI=y:

(1) function gic_handle_irq has bti in its head, so we adjust rec->ip+=5 to last nop
ffffffc0080100e0: d53cd042 mrs x2, tpidr_el2
...
ffffffc0080100f0: d503201f nop //__mcount_loc tells the rec->ip
ffffffc0080100f4: d503201f nop
ffffffc0080100f8: d503201f nop

ffffffc0080100fc <gic_handle_irq>:
ffffffc0080100fc: d503245f bti c
ffffffc008010100: d503201f nop
ffffffc008010104: d503201f nop //we adjust origin rec->ip+5 to here
ffffffc008010108: d503233f paciasp
(2) name_to_dev_t.part.0 do not has bti in its head, so we should adjust rec->ip+=4 to last nop
ffff8000080137d4: d503201f nop
ffff8000080137d8: d503201f nop
ffff8000080137dc: d503201f nop

ffff8000080137e0 <name_to_dev_t.part.0>:
ffff8000080137e0: d503201f nop
ffff8000080137e4: d503201f nop
ffff8000080137e8: d503233f paciasp

So at this time we have no idea to identify rec->ip for each tracable function.

we are looking forward to follow-up discussions.

References:
[1] https://developer.arm.com/documentation/100076/0100/a64-instruction-set-reference/a64-general-instructions/bti
[2] https://lore.kernel.org/linux-arm-kernel/20200109142736.1122-1-cj.chengjian@huawei.com/

Cheng Jian (4):
arm64: introduce aarch64_insn_gen_load_literal
arm64/ftrace: introduce ftrace dynamic trampoline entrances
arm64/ftrace: support dynamically allocated trampolines
arm64/ftrace: implement long jump for dynamic trampolines

arch/arm64/Makefile | 2 +-
arch/arm64/include/asm/ftrace.h | 10 +-
arch/arm64/include/asm/insn.h | 6 +
arch/arm64/include/asm/module.h | 9 +
arch/arm64/kernel/entry-ftrace.S | 88 ++++++--
arch/arm64/kernel/ftrace.c | 366 ++++++++++++++++++++++++++++---
arch/arm64/kernel/module-plts.c | 50 +++++
arch/arm64/lib/insn.c | 49 +++++
8 files changed, 532 insertions(+), 48 deletions(-)

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2.25.1