Re: [PATCHv3] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

[Jean-Jacques Hiblot]

2017-02-09 17:29 GMT+01:00 Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx>:
> On Tue, Feb 07, 2017 at 10:57:55PM +0000, Abel Vesa wrote:
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>> +
>> +.macro __ftrace_regs_caller
>> +
>> +     add     ip, sp, #4      @ move in IP the value of SP as it was
>> +                             @ before the push {lr} of the mcount mechanism
>> +     stmdb   sp!, {ip,lr,pc}
>> +     stmdb   sp!, {r0-r11,lr}
>> +
>> +     @ stack content at this point:
>> +     @ 0  4          44    48   52       56   60   64
>> +     @ R0 | R1 | ... | R11 | LR | SP + 4 | LR | PC | previous LR |
>
> How important is this to be close to "struct pt_regs" ?  Do we care about
> r12 being "wrong" ?  The other issue is that pt_regs is actually 72
> bytes in size, not 68 bytes.  So, does that mean we end up inappropriately
> leaking some of the kernel stack to userspace through ftrace?
>
> It's possible to save all the registers like this if we need to provide
> a complete picture of the register set at function entry:
>
>         str     ip, [sp, #-16]!
>         add     ip, sp, #20
>         stmia   sp, {ip, lr, pc}
>         stmdb   sp!, {r0 - r11}
>
> However, is that even correct - don't we want pt_regs' LR and PC to be
> related to the function call itself?  The "previous LR" as you describe
> it is where the called function (the one that is being traced) will
> return to.  The current LR at this point is the address within the
> traced function.  So actually I think this is more strictly correct, if
> I'm understanding the intention here correctly:
>
>         str     ip, [sp, #S_IP - PT_REGS_SIZE]! @ save current IP
>         ldr     ip, [sp, #PT_REGS_SIZE - S_IP]  @ get LR at traced function entry
>         str     lr, [sp, #S_PC - S_IP]          @ save current LR as PC
>         str     ip, [sp, #S_LR - S_IP]          @ save traced function return
>         add     ip, sp, #PT_REGS_SIZE - S_IP + 4
>         str     ip, [sp, #S_SP - SP_IP]         @ save stack pointer at function entry
>         stmdb   sp!, {r0 - r11}
>         @ clear CPSR and old_r0 words
>         mov     r3, #0
>         str     r3, [sp, #S_PSR]
>         str     r3, [sp, #S_OLD_R0]
>
> However, that has the side effect of misaligning the stack (the stack
> needs to be aligned to 8 bytes).  So, if we decide we don't care about
> the saved LR value (except as a mechanism to preserve it across the
> call into the ftrace code):
>
>         str     ip, [sp, #S_IP - PT_REGS_SIZE + 4]!
>         str     lr, [sp, #S_PC - S_IP]
>         ldr     lr, [sp, #PT_REGS_SIZE - 4 - S_IP]
>         add     ip, sp, #PT_REGS_SIZE - S_IP
>         stmib   sp, {ip, lr}
>         stmdb   sp!, {r0 - r11}
>         @ clear CPSR and old_r0 words
>         mov     r3, #0
>         str     r3, [sp, #S_PSR]
>         str     r3, [sp, #S_OLD_R0]
>
> and the return would be:
>
>         ldmia   sp, {r0 - pc}
>
> That all said - maybe someone from the ftrace community can comment on
> how much of pt_regs is actually necessary here?

I would suggest the following:
r0-r11: filled with current values.
r12 :  the value of r12 doesn't matter (Intra-procedure call scratch
reg), we can either save it or not.
r13 - sp: the value as it was when the instrumented function was
entered. in the mcount case, it's the current sp value - 4, otherwise
it'f sp -4
r14 - lr: the value as it was when the instrumented function was
entered. first element in stack or available in frame depending on
GCC's version (mcount vs __gnu_mcount_nc)
r15 - pc : the address after the modified instruction (value of lr
when the ftrace caller is entered)

I don't think we need CSPR and ORIG_r0.

>
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