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Message-ID: <20170319210907.GA11479@nuc>
Date: Sun, 19 Mar 2017 21:09:07 +0000
From: Abel Vesa <abelvesa@...il.com>
To: Rabin Vincent <rabin@....in>
Cc: Abel Vesa <abelvesa@...ux.com>, robin.murphy@....com,
jjhiblot@...phandler.com, nicstange@...il.com,
Russell King <linux@...linux.org.uk>,
Steven Rostedt <rostedt@...dmis.org>,
Ingo Molnar <mingo@...hat.com>, pmladek@...e.com,
mhiramat@...nel.org, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5] arm: ftrace: Adds support for
CONFIG_DYNAMIC_FTRACE_WITH_REGS
On Sat, Mar 04, 2017 at 02:02:17PM +0100, Rabin Vincent wrote:
> On Sat, Mar 04, 2017 at 12:51:12AM +0000, Abel Vesa wrote:
> > diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
> > index c73c403..93f9abb 100644
> > --- a/arch/arm/kernel/entry-ftrace.S
> > +++ b/arch/arm/kernel/entry-ftrace.S
> > @@ -92,12 +92,78 @@
> > 2: mcount_exit
> > .endm
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +
> > +.macro __ftrace_regs_caller
> > +
> > + sub sp, sp, #8 @ space for CPSR and OLD_R0 (not used)
> > +
> > + add ip, sp, #12 @ move in IP the value of SP as it was
> > + @ before the push {lr} of the mcount mechanism
> > + stmdb sp!, {ip,lr,pc}
>
> This doesn't build with CONFIG_THUMB2_KERNEL:
>
> entry-ftrace.S:285: Error: PC not allowed in register list -- `stmdb sp!,{ip,lr,pc}'
>
> Saving PC in STMDB is prohibited in Thumb and deprecated in ARM.
>
> > + stmdb sp!, {r0-r11,lr}
> > +
> > + ldr r0, [sp, #S_LR] @ replace PC with LR
> > + str r0, [sp, #S_PC] @ into pt_regs
> > +
> > + ldr r1, [sp, #PT_REGS_SIZE] @ replace new LR with
> > + str r1, [sp, #S_LR] @ previous LR into pt_regs
> > +
> > + @ stack content at this point:
> > + @ 0 4 48 52 56 60 64 68 72
> > + @ R0 | R1 | ... | LR | SP + 4 | previous LR | LR | PSR | OLD_R0 | previous LR |
>
> So this code only adjust the SP by 72 bytes. Since the stack is not
> aligned to 8 bytes at entry to mcount, this code calls into C functions
> with a misaligned stack. This is a violation of the procedure call
> standard.
>
> Also, I believe this should have unwind annotations to indicate where it
> saved the registers. See mcount_enter for an example.
>
> > +
> > + mov r3, sp @ struct pt_regs*
> > + ldr r2, =function_trace_op
> > + ldr r2, [r2] @ pointer to the current
> > + @ function tracing op
> > + ldr r1, [sp, #PT_REGS_SIZE] @ lr of instrumented func
> > + mcount_adjust_addr r0, lr @ instrumented function
> > +
> > + .globl ftrace_regs_call
> > +ftrace_regs_call:
> > + bl ftrace_stub
> > +
> > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > + .globl ftrace_graph_regs_call
> > +ftrace_graph_regs_call:
> > + mov r0, r0
> > +#endif
> > + @ pop saved regs
> > + ldmia sp, {r0-r15}
>
> This doesn't build either:
>
> entry-ftrace.S:285: Error: LR and PC should not both be in register list -- `ldmia sp,{r0-r15}'
>
> Restoring LR and PC together is prohibited in Thumb and deprecated in
> ARM. It's the same case with SP.
>
We could do it in separate instructions, something like:
@ pop saved regs
ldmia sp!, {r0-r12}
ldr ip, [sp, #8]
ldr lr, [sp, #4]
ldr sp, [sp, #0]
mov pc, ip
Same goes for saving the regs, except I need to replace the LR stored outside of ftrace_regs_caller
with OLD_R0 so that the stack would become aligned to 8 bytes.
> > +.endm
> > +
> > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > +.macro __ftrace_graph_regs_caller
> > +
> > + sub r0, fp, #4 @ lr of instrumented routine (parent)
> > +
> > + @ called from __ftrace_regs_caller
> > + ldr r1, [sp, #S_PC] @ instrumented routine (func)
> > + mcount_adjust_addr r1, r1
> > +
> > + mov r2, fp @ frame pointer
> > + bl prepare_ftrace_return
> > +
> > + @ pop registers saved in ftrace_regs_caller
> > + ldmia sp, {r0-r15}
>
> This doesn't get built on CONFIG_THUMB2_KERNEL since there's no suppport
> for CONFIG_FUNCTION_GRAPH_TRACER there, but this is also using
> deprecated operands.
>
> > +.endm
> > +#endif
> > +#endif
> ...
> > diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
> > index 3f17594..f165265 100644
> > --- a/arch/arm/kernel/ftrace.c
> > +++ b/arch/arm/kernel/ftrace.c
> > @@ -29,11 +29,6 @@
> > #endif
> >
> > #ifdef CONFIG_DYNAMIC_FTRACE
> > -#ifdef CONFIG_OLD_MCOUNT
> > -#define OLD_MCOUNT_ADDR ((unsigned long) mcount)
> > -#define OLD_FTRACE_ADDR ((unsigned long) ftrace_caller_old)
> > -
> > -#define OLD_NOP 0xe1a00000 /* mov r0, r0 */
> >
> > static int __ftrace_modify_code(void *data)
> > {
> > @@ -51,6 +46,12 @@ void arch_ftrace_update_code(int command)
> > stop_machine(__ftrace_modify_code, &command, NULL);
> > }
> >
> > +#ifdef CONFIG_OLD_MCOUNT
> > +#define OLD_MCOUNT_ADDR ((unsigned long) mcount)
> > +#define OLD_FTRACE_ADDR ((unsigned long) ftrace_caller_old)
> > +
> > +#define OLD_NOP 0xe1a00000 /* mov r0, r0 */
> > +
> > static unsigned long ftrace_nop_replace(struct dyn_ftrace *rec)
>
> This chunk above fixes the problem which Nicolai pointed out earlier. Since
> it's a bug fix for the current dynamic ftrace code, it should really be
> submitted as a separate patch. Feel free to use some variant of this
> commit message which I wrote when I ran into this yesterday:
>
> ARM: ftrace: fix dynamic ftrace with DEBUG_RODATA and !FRAME_POINTER
>
> The support for dynamic ftrace with CONFIG_DEBUG_RODATA involves
> overriding the weak arch_ftrace_update_code() with a variant which makes
> the kernel text writable around the patching.
>
> This override was however added under the CONFIG_OLD_MCOUNT ifdef, and
> CONFIG_OLD_MCOUNT is only enabled if frame pointers are enabled.
>
> This leads to non-functional dynamic ftrace (ftrace triggers a
> WARN_ON()) when CONFIG_DEBUG_RODATA is enabled and CONFIG_FRAME_POINTER
> is not.
>
> Move the override out of that ifdef and into the CONFIG_DYNAMIC_FTRACE
> ifdef where it belongs.
>
> Fixes: 80d6b0c2eed2a ("ARM: mm: allow text and rodata sections to be read-only")
>
Sent as a separate patch already.
> > {
> > return rec->arch.old_mcount ? OLD_NOP : NOP;
> > @@ -139,6 +140,15 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
> >
> > ret = ftrace_modify_code(pc, 0, new, false);
> >
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>
> This code (and the rest of your additions to this file below), are already
> under CONFIG_DYNAMIC_FTRACE and afaics CONFIG_DYNAMIC_FTRACE_WITH_REGS is
> always enabled when the former is enabled, so could you please drop
> these ifdefs?
>
> The code in entry-ftrace.S under CONFIG_DYNAMIC_FTRACE_WITH_REGS would
> also benefit from just using CONFIG_DYNAMIC_FTRACE everywhere instead.
I believe this is incorrect, I think it is worth being compiled in only if
CONFIG_DYNAMIC_FTRACE_WITH_REGS, but this is dependent on CONFIG_DYNAMIC_FTRACE
and the first one is not necessarily enabled if the second one is. Otherwise, what's
the point of having two CONFIGs ?
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