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c2c6b27b5a
When arm64's stack unwinder encounters an exception boundary, it uses the pt_regs::stackframe created by the entry code, which has a copy of the PC and FP at the time the exception was taken. The unwinder doesn't know anything about pt_regs, and reports the PC from the stackframe, but does not report the LR. The LR is only guaranteed to contain the return address at function call boundaries, and can be used as a scratch register at other times, so the LR at an exception boundary may or may not be a legitimate return address. It would be useful to report the LR value regardless, as it can be helpful when debugging, and in future it will be helpful for reliable stacktrace support. This patch changes the way we unwind across exception boundaries, allowing both the PC and LR to be reported. The entry code creates a frame_record_meta structure embedded within pt_regs, which the unwinder uses to find the pt_regs. The unwinder can then extract pt_regs::pc and pt_regs::lr as two separate unwind steps before continuing with a regular walk of frame records. When a PC is unwound from pt_regs::lr, dump_backtrace() will log this with an "L" marker so that it can be identified easily. For example, an unwind across an exception boundary will appear as follows: | el1h_64_irq+0x6c/0x70 | _raw_spin_unlock_irqrestore+0x10/0x60 (P) | __aarch64_insn_write+0x6c/0x90 (L) | aarch64_insn_patch_text_nosync+0x28/0x80 ... with a (P) entry for pt_regs::pc, and an (L) entry for pt_regs:lr. Note that the LR may be stale at the point of the exception, for example, shortly after a return: | el1h_64_irq+0x6c/0x70 | default_idle_call+0x34/0x180 (P) | default_idle_call+0x28/0x180 (L) | do_idle+0x204/0x268 ... where the LR points a few instructions before the current PC. This plays nicely with all the other unwind metadata tracking. With the ftrace_graph profiler enabled globally, and kretprobes installed on generic_handle_domain_irq() and do_interrupt_handler(), a backtrace triggered by magic-sysrq + L reports: | Call trace: | show_stack+0x20/0x40 (CF) | dump_stack_lvl+0x60/0x80 (F) | dump_stack+0x18/0x28 | nmi_cpu_backtrace+0xfc/0x140 | nmi_trigger_cpumask_backtrace+0x1c8/0x200 | arch_trigger_cpumask_backtrace+0x20/0x40 | sysrq_handle_showallcpus+0x24/0x38 (F) | __handle_sysrq+0xa8/0x1b0 (F) | handle_sysrq+0x38/0x50 (F) | pl011_int+0x460/0x5a8 (F) | __handle_irq_event_percpu+0x60/0x220 (F) | handle_irq_event+0x54/0xc0 (F) | handle_fasteoi_irq+0xa8/0x1d0 (F) | generic_handle_domain_irq+0x34/0x58 (F) | gic_handle_irq+0x54/0x140 (FK) | call_on_irq_stack+0x24/0x58 (F) | do_interrupt_handler+0x88/0xa0 | el1_interrupt+0x34/0x68 (FK) | el1h_64_irq_handler+0x18/0x28 | el1h_64_irq+0x6c/0x70 | default_idle_call+0x34/0x180 (P) | default_idle_call+0x28/0x180 (L) | do_idle+0x204/0x268 | cpu_startup_entry+0x3c/0x50 (F) | rest_init+0xe4/0xf0 | start_kernel+0x744/0x750 | __primary_switched+0x88/0x98 Signed-off-by: Mark Rutland <mark.rutland@arm.com> Reviewed-by: Mark Brown <broonie@kernel.org> Reviewed-by: Miroslav Benes <mbenes@suse.cz> Reviewed-by: Puranjay Mohan <puranjay12@gmail.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: Josh Poimboeuf <jpoimboe@kernel.org> Cc: Kalesh Singh <kaleshsingh@google.com> Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com> Cc: Marc Zyngier <maz@kernel.org> Cc: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20241017092538.1859841-11-mark.rutland@arm.com Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> |
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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 reStructuredText 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.