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SubjectRe: [RFC PATCH] ARM: UNWINDER_FRAME_POINTER implementation for Clang
On Thu, Aug 1, 2019 at 4:10 PM 'Nathan Huckleberry' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> The stackframe setup when compiled with clang is different.
> Since the stack unwinder expects the gcc stackframe setup it
> fails to print backtraces. This patch adds support for the
> clang stackframe setup.
>
> Cc: clang-built-linux@googlegroups.com
> Suggested-by: Tri Vo <trong@google.com>
> Signed-off-by: Nathan Huckleberry <nhuck@google.com>

Thanks for the patch! This is something definitely useful to have
implemented with Clang. Some initial thoughts below:

> ---
> arch/arm/Kconfig.debug | 4 +-
> arch/arm/Makefile | 2 +-
> arch/arm/lib/backtrace.S | 134 ++++++++++++++++++++++++++++++++++++---
> 3 files changed, 128 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index 85710e078afb..92fca7463e21 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -56,7 +56,7 @@ choice
>
> config UNWINDER_FRAME_POINTER
> bool "Frame pointer unwinder"
> - depends on !THUMB2_KERNEL && !CC_IS_CLANG
> + depends on !THUMB2_KERNEL
> select ARCH_WANT_FRAME_POINTERS
> select FRAME_POINTER
> help
> @@ -1872,7 +1872,7 @@ config DEBUG_UNCOMPRESS
> When this option is set, the selected DEBUG_LL output method
> will be re-used for normal decompressor output on multiplatform
> kernels.
> -
> +
>
> config UNCOMPRESS_INCLUDE
> string
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index c3624ca6c0bc..a593d9c4e18a 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -36,7 +36,7 @@ KBUILD_CFLAGS += $(call cc-option,-mno-unaligned-access)
> endif
>
> ifeq ($(CONFIG_FRAME_POINTER),y)
> -KBUILD_CFLAGS +=-fno-omit-frame-pointer -mapcs -mno-sched-prolog
> +KBUILD_CFLAGS +=-fno-omit-frame-pointer $(call cc-option,-mapcs,) $(call cc-option,-mno-sched-prolog,)
> endif
>
> ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
> diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
> index 1d5210eb4776..fd64eec9f6ae 100644
> --- a/arch/arm/lib/backtrace.S
> +++ b/arch/arm/lib/backtrace.S
> @@ -14,10 +14,7 @@
> @ fp is 0 or stack frame
>
> #define frame r4
> -#define sv_fp r5
> -#define sv_pc r6

It looks like sv_fp and sv_pc have the same values for both GCC and
Clang, maybe they don't need to be moved here?

> #define mask r7
> -#define offset r8

So GCC has an offset while Clang has sv_lr.

>
> ENTRY(c_backtrace)
>
> @@ -25,7 +22,8 @@ ENTRY(c_backtrace)
> ret lr
> ENDPROC(c_backtrace)
> #else
> - stmfd sp!, {r4 - r8, lr} @ Save an extra register so we have a location...
> + stmfd sp!, {r4 - r8, fp, lr} @ Save an extra register

Not having a preprocessor guard here makes it seem like GCC will
always have to move the additional register, even if it's not using
it?

> + @ so we have a location..
> movs frame, r0 @ if frame pointer is zero
> beq no_frame @ we have no stack frames
>
> @@ -35,11 +33,119 @@ ENDPROC(c_backtrace)
> THUMB( orreq mask, #0x03 )
> movne mask, #0 @ mask for 32-bit
>
> -1: stmfd sp!, {pc} @ calculate offset of PC stored
> - ldr r0, [sp], #4 @ by stmfd for this CPU
> - adr r1, 1b
> - sub offset, r0, r1
>
> +#if defined(CONFIG_CC_IS_CLANG)

#ifdef CONFIG_CC_IS_CLANG

I'd only use `#if defined(foo)` if there were 2 or more things I
needed to check: `#if defined(foo) || defined(bar)`.

> +/*
> + * Clang does not store pc or sp in function prologues
> + * so we don't know exactly where the function
> + * starts.
> + * We can treat the current frame's lr as the saved pc and the
> + * preceding frame's lr as the lr, but we can't
> + * trace the most recent call.
> + * Inserting a false stack frame allows us to reference the
> + * function called last in the stacktrace.
> + * If the call instruction was a bl we can look at the callers
> + * branch instruction to calculate the saved pc.
> + * We can recover the pc in most cases, but in cases such as
> + * calling function pointers we cannot. In this
> + * case, default to using the lr. This will be
> + * some address in the function, but will not
> + * be the function start.
> + * Unfortunately due to the stack frame layout we can't dump
> + * r0 - r3, but these are less frequently saved.
> + *
> + * Stack frame layout:
> + * <larger addresses>
> + * saved lr
> + * frame => saved fp
> + * optionally saved caller registers (r4 - r10)
> + * optionally saved arguments (r0 - r3)
> + * <top of stack frame>
> + * <smaller addressses>

s/addressses/addresses/

> + *
> + * Functions start with the following code sequence:
> + * corrected pc => stmfd sp!, {..., fp, lr}
> + * add fp, sp, #x
> + * stmfd sp!, {r0 - r3} (optional)
> + */
> +#define sv_fp r5
> +#define sv_pc r6
> +#define sv_lr r8
> + add frame, sp, #20 @ switch to false frame
> +for_each_frame: tst frame, mask @ Check for address exceptions
> + bne no_frame
> +
> +1001: ldr sv_pc, [frame, #4] @ get saved 'pc'
> +1002: ldr sv_fp, [frame, #0] @ get saved fp

These two sections seem to differ between GCC and Clang only for the
offsets. Could these be made into preprocessor defines and more code
shared?

> +
> + teq sv_fp, #0 @ make sure next frame exists
> + beq no_frame
> +
> +1003: ldr sv_lr, [sv_fp, #4] @ get saved lr from next frame
> +
> + //try to find function start

Use either /* c89 comments */ or @arm assembler comments.

> + ldr r0, [sv_lr, #-4] @ get call instruction
> + ldr r3, .Ldsi+8
> + and r2, r3, r0 @ is this a bl call
> + teq r2, r3
> + bne finished_setup @ give up if it's not
> + and r0, #0xffffff @ get call offset 24-bit int
> + lsl r0, r0, #8 @ sign extend offset
> + asr r0, r0, #8
> + ldr sv_pc, [sv_fp, #4] @ get lr address
> + add sv_pc, sv_pc, #-4 @ get call instruction address
> + add sv_pc, sv_pc, #8 @ take care of prefetch
> + add sv_pc, sv_pc, r0, lsl #2 @ find function start
> + b finished_setup

Do we need an explicit branch to this label? Wouldn't we just fall
through to it?j

> +
> +finished_setup:
> +
> + bic sv_pc, sv_pc, mask @ mask PC/LR for the mode
> +
> +1004: mov r0, sv_pc
> +
> + mov r1, sv_lr
> + mov r2, frame
> + bic r1, r1, mask @ mask PC/LR for the mode
> + bl dump_backtrace_entry
> +
> +1005: ldr r1, [sv_pc, #0] @ if stmfd sp!, {..., fp, lr}
> + ldr r3, .Ldsi @ instruction exists,
> + teq r3, r1, lsr #11
> + ldr r0, [frame] @ locals are stored in
> + @ the preceding frame
> + subeq r0, r0, #4
> + bleq dump_backtrace_stm @ dump saved registers
> +
> + teq sv_fp, #0 @ zero saved fp means
> + beq no_frame @ no further frames
> +
> + cmp sv_fp, frame @ next frame must be
> + mov frame, sv_fp @ above the current frame
> + bhi for_each_frame
> +
> +1006: adr r0, .Lbad
> + mov r1, frame
> + bl printk
> +no_frame: ldmfd sp!, {r4 - r8, fp, pc}
> +ENDPROC(c_backtrace)
> + .pushsection __ex_table,"a"
> + .align 3
> + .long 1001b, 1006b
> + .long 1002b, 1006b
> + .long 1003b, 1006b
> + .long 1004b, 1006b
> + .popsection
> +
> +.Lbad: .asciz "Backtrace aborted due to bad frame pointer <%p>\n"
> + .align

Probably don't need to duplicate the above (up to ENDPROC), but the
below hunk looks compiler specific.

> +.Ldsi: .word 0xe92d4800 >> 11 @ stmfd sp!, {... fp, lr}
> + .word 0xe92d0000 >> 11 @ stmfd sp!, {}
> + .word 0x0b000000 @ bl if these bits are set
> +
> +ENDPROC(c_backtrace)

Duplicate ENDPROC?

> +
> +#else
> /*
> * Stack frame layout:
> * optionally saved caller registers (r4 - r10)
> @@ -55,6 +161,15 @@ ENDPROC(c_backtrace)
> * stmfd sp!, {r0 - r3} (optional)
> * corrected pc => stmfd sp!, {..., fp, ip, lr, pc}
> */
> +#define sv_fp r5
> +#define sv_pc r6
> +#define offset r8
> +
> +1: stmfd sp!, {pc} @ calculate offset of PC stored
> + ldr r0, [sp], #4 @ by stmfd for this CPU
> + adr r1, 1b
> + sub offset, r0, r1
> +
> for_each_frame: tst frame, mask @ Check for address exceptions
> bne no_frame
>
> @@ -98,7 +213,7 @@ for_each_frame: tst frame, mask @ Check for address exceptions
> 1006: adr r0, .Lbad
> mov r1, frame
> bl printk
> -no_frame: ldmfd sp!, {r4 - r8, pc}
> +no_frame: ldmfd sp!, {r4 - r8, fp, pc}

More work for GCC...

> ENDPROC(c_backtrace)
>
> .pushsection __ex_table,"a"
> @@ -115,3 +230,4 @@ ENDPROC(c_backtrace)
> .word 0xe92d0000 >> 11 @ stmfd sp!, {}
>
> #endif
> +#endif

It would be nice to put comments on the end of these #endif's what
condition they're terminating:

#endif /* CONFIG_CC_IS_CLANG
#endif /* !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK) */
Maybe also the #else's above.

Will send more thoughts tomorrow/throughout the week.
--
Thanks,
~Nick Desaulniers
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