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Mark Rutland c607ab4f91 arm64: stacktrace: don't trace arch_stack_walk() 
We recently converted arm64 to use arch_stack_walk() in commit:

  5fc57df2f6 ("arm64: stacktrace: Convert to ARCH_STACKWALK")

The core stacktrace code expects that (when tracing the current task)
arch_stack_walk() starts a trace at its caller, and does not include
itself in the trace. However, arm64's arch_stack_walk() includes itself,
and so traces include one more entry than callers expect. The core
stacktrace code which calls arch_stack_walk() tries to skip a number of
entries to prevent itself appearing in a trace, and the additional entry
prevents skipping one of the core stacktrace functions, leaving this in
the trace unexpectedly.

We can fix this by having arm64's arch_stack_walk() begin the trace with
its caller. The first value returned by the trace will be
__builtin_return_address(0), i.e. the caller of arch_stack_walk(). The
first frame record to be unwound will be __builtin_frame_address(1),
i.e. the caller's frame record. To prevent surprises, arch_stack_walk()
is also marked noinline.

While __builtin_frame_address(1) is not safe in portable code, local GCC
developers have confirmed that it is safe on arm64. To find the caller's
frame record, the builtin can safely dereference the current function's
frame record or (in theory) could stash the original FP into another GPR
at function entry time, neither of which are problematic.

Prior to this patch, the tracing code would unexpectedly show up in
traces of the current task, e.g.

| # cat /proc/self/stack
| [<0>] stack_trace_save_tsk+0x98/0x100
| [<0>] proc_pid_stack+0xb4/0x130
| [<0>] proc_single_show+0x60/0x110
| [<0>] seq_read_iter+0x230/0x4d0
| [<0>] seq_read+0xdc/0x130
| [<0>] vfs_read+0xac/0x1e0
| [<0>] ksys_read+0x6c/0xfc
| [<0>] __arm64_sys_read+0x20/0x30
| [<0>] el0_svc_common.constprop.0+0x60/0x120
| [<0>] do_el0_svc+0x24/0x90
| [<0>] el0_svc+0x2c/0x54
| [<0>] el0_sync_handler+0x1a4/0x1b0
| [<0>] el0_sync+0x170/0x180

After this patch, the tracing code will not show up in such traces:

| # cat /proc/self/stack
| [<0>] proc_pid_stack+0xb4/0x130
| [<0>] proc_single_show+0x60/0x110
| [<0>] seq_read_iter+0x230/0x4d0
| [<0>] seq_read+0xdc/0x130
| [<0>] vfs_read+0xac/0x1e0
| [<0>] ksys_read+0x6c/0xfc
| [<0>] __arm64_sys_read+0x20/0x30
| [<0>] el0_svc_common.constprop.0+0x60/0x120
| [<0>] do_el0_svc+0x24/0x90
| [<0>] el0_svc+0x2c/0x54
| [<0>] el0_sync_handler+0x1a4/0x1b0
| [<0>] el0_sync+0x170/0x180

Erring on the side of caution, I've given this a spin with a bunch of
toolchains, verifying the output of /proc/self/stack and checking that
the assembly looked sound. For GCC (where we require version 5.1.0 or
later) I tested with the kernel.org crosstool binares for versions
5.5.0, 6.4.0, 6.5.0, 7.3.0, 7.5.0, 8.1.0, 8.3.0, 8.4.0, 9.2.0, and
10.1.0. For clang (where we require version 10.0.1 or later) I tested
with the llvm.org binary releases of 11.0.0, and 11.0.1.

Fixes: 5fc57df2f6 ("arm64: stacktrace: Convert to ARCH_STACKWALK")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chen Jun <chenjun102@huawei.com>
Cc: Marco Elver <elver@google.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: <stable@vger.kernel.org> # 5.10.x
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210319184106.5688-1-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
2021-03-22 12:42:49 +00:00
arch arm64: stacktrace: don't trace arch_stack_walk() 2021-03-22 12:42:49 +00:00
block block/bfq: update comments and default value in docs for fifo_expire 2021-03-02 11:25:38 -07:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto Keyrings miscellany 2021-02-23 16:09:23 -08:00
Documentation block-5.12-2021-03-05 2021-03-05 12:59:37 -08:00
drivers perf/arm_dmc620_pmu: Fix error return code in dmc620_pmu_device_probe() 2021-03-12 11:30:31 +00:00
fs pstore fixes for v5.12-rc2 2021-03-05 17:21:25 -08:00
include arm64: mte: Map hotplugged memory as Normal Tagged 2021-03-10 10:56:46 +00:00
init kbuild: fix UNUSED_KSYMS_WHITELIST for Clang LTO 2021-02-28 15:19:21 +09:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel io_uring-5.12-2021-03-05 2021-03-05 12:44:43 -08:00
lib Merge branch 'kmap-conversion-for-5.12' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux 2021-03-01 11:24:18 -08:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm arm64: mte: Map hotplugged memory as Normal Tagged 2021-03-10 10:56:46 +00:00
net io_uring-worker.v3-2021-02-25 2021-02-27 08:29:02 -08:00
samples treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
scripts gcc-plugins fixes for v5.12-rc2 2021-03-05 17:23:03 -08:00
security Keyrings miscellany 2021-02-23 16:09:23 -08:00
sound ALSA: hda/realtek: Apply dual codec quirks for MSI Godlike X570 board 2021-03-03 15:25:03 +01:00
tools kselftest: arm64: Fix exit code of sve-ptrace 2021-03-10 10:58:11 +00:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS RISC-V Patches for the 5.12 Merge Window 2021-02-26 10:28:35 -08:00
Makefile Linux 5.12-rc2 2021-03-05 17:33:41 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.