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

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Date:   Tue, 15 Jun 2021 18:52:01 -0700
From:   Suraj Jitindar Singh <sjitindarsingh@...il.com>
To:     madvenka@...ux.microsoft.com, broonie@...nel.org,
        mark.rutland@....com, jpoimboe@...hat.com, ardb@...nel.org,
        nobuta.keiya@...itsu.com, catalin.marinas@....com, will@...nel.org,
        jmorris@...ei.org, pasha.tatashin@...een.com, jthierry@...hat.com,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 2/2] arm64: Create a list of SYM_CODE functions,
 check return PC against list

On Wed, 2021-05-26 at 16:49 -0500, madvenka@...ux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@...ux.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