Re: [RFC PATCH v5 2/2] arm64: Create a list of SYM_CODE functions, check return PC against list

Suraj Jitindar Singh
2021-06-16T01:52:06Z

On Wed, 2021-05-26 at 16:49 -0500, madvenka@linux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>
> The unwinder should check if the return PC falls in any function that
> is considered unreliable from an unwinding perspective. If it does,
> mark the stack trace unreliable.
>

[snip]

Correct me if I'm wrong, but do you not need to move the final frame
check to before the unwinder_is_unreliable() call?

Userland threads which have ret_from_fork as the last entry on the
stack will always be marked unreliable as they will always have a
SYM_CODE entry on their stack (the ret_from_fork).

Also given that this means the last frame has been reached and as such
there's no more unwinding to do, I don't think we care if the last pc
is a code address.

- Suraj

> *
> @@ -133,7 +236,20 @@ int notrace unwind_frame(struct task_struct
> *tsk, struct stackframe *frame)
> * - Foreign code (e.g. EFI runtime services)
> * - Procedure Linkage Table (PLT) entries and veneer
> functions
> */
> - if (!__kernel_text_address(frame->pc))
> + if (!__kernel_text_address(frame->pc)) {
> + frame->reliable = false;
> + return 0;
> + }
> +
> + /*
> + * If the final frame has been reached, there is no more
> unwinding
> + * to do. There is no need to check if the return PC is
> considered
> + * unreliable by the unwinder.
> + */
> + if (!frame->fp)
> + return 0;

if (frame->fp == (unsigned long)task_pt_regs(tsk)->stackframe)
return -ENOENT;

> +
> + if (unwinder_is_unreliable(frame->pc))
> frame->reliable = false;
>
> return 0;
> diff --git a/arch/arm64/kernel/vmlinux.lds.S
> b/arch/arm64/kernel/vmlinux.lds.S
> index 7eea7888bb02..32e8d57397a1 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -103,6 +103,12 @@ jiffies = jiffies_64;
> #define TRAMP_TEXT
> #endif
>
> +#define SYM_CODE_FUNCTIONS \
> + . = ALIGN(16); \
> + __sym_code_functions_start = .; \
> + KEEP(*(sym_code_functions)) \
> + __sym_code_functions_end = .;
> +
> /*
> * The size of the PE/COFF section that covers the kernel image,
> which
> * runs from _stext to _edata, must be a round multiple of the
> PE/COFF
> @@ -218,6 +224,7 @@ SECTIONS
> CON_INITCALL
> INIT_RAM_FS
> *(.init.altinstructions .init.bss) /* from the
> EFI stub */
> + SYM_CODE_FUNCTIONS
> }
> .exit.data : {
> EXIT_DATA