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SubjectRe: [RFC PATCH] ARM: UNWINDER_FRAME_POINTER implementation for Clang
You're right. Would pushing an extra register be an adequate fix?

On Fri, Aug 2, 2019 at 7:24 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 02/08/2019 00:10, Nathan Huckleberry 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>
> > ---
> > 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
> > #define mask r7
> > -#define offset r8
> >
> > 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
>
> Note that the Procedure Call Standard for EABI requires that SP be
> 8-byte-aligned at a public interface. Pushing an odd number of registers
> here looks like it will make the subsequent calls to dump_backtrace_*
> and printk violate that condition.
>
> Robin.
>
> > + @ 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)
> > +/*
> > + * 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>
> > + *
> > + * 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
> > +
> > + 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
> > + 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
> > +
> > +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
> > +.Ldsi: .word 0xe92d4800 >> 11 @ stmfd sp!, {... fp, lr}
> > + .word 0xe92d0000 >> 11 @ stmfd sp!, {}
> > + .word 0x0b000000 @ bl if these bits are set
> > +
> > +ENDPROC(c_backtrace)
> > +
> > +#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}
> > ENDPROC(c_backtrace)
> >
> > .pushsection __ex_table,"a"
> > @@ -115,3 +230,4 @@ ENDPROC(c_backtrace)
> > .word 0xe92d0000 >> 11 @ stmfd sp!, {}
> >
> > #endif
> > +#endif
> >

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