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?
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