2017-02-09 17:29 GMT+01:00 Russell King - ARM Linux <[email protected]>: > 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. > > -- > RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ > FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up > according to speedtest.net.
