Messages in this thread | | | From | Jean-Jacques Hiblot <> | Date | Fri, 10 Feb 2017 14:57:38 +0100 | Subject | Re: [PATCHv3] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS |
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2017-02-10 13:03 GMT+01:00 Abel Vesa <abelvesa@gmail.com>: > On Fri, Feb 10, 2017 at 11:36:12AM +0100, Jean-Jacques Hiblot wrote: >> 2017-02-09 17:29 GMT+01:00 Russell King - ARM Linux <linux@armlinux.org.uk>: >> > 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) >> > So basically you're saying: save all regs, r0 through r15, except r12. > Based on that, I think it's easier to save all of them and then restore > all of them except r12. Plus, you have to take into consideration all > the possible users of ftrace with regs. At some point some implementation > of ftrace_regs_call will probably need the value from r12. >> I don't think we need CSPR and ORIG_r0. > I think we do. As I said before, because PT_REGS is 72 and some function > might (in the future) make use of CSPR or ORIG_r0, to ensure there is no > stack corruption taking place, we have to provide whole pt_regs, that is > 72 (including CSPR and ORIG_r0). Plus, the stack alignment thing Russell > mentioned would be fixed. You're right for the size of the structure. For the content, I don't think we need all of them but it won't hurt to save more than necessary. > > The only problem I don't have a solution for at this point is OLD_LR (or previous LR > as it is called in this patch). If we take the approach described earlier,
previous LR is the lr when the instrumented function is entered, it should be stored in pt_regs as r14
> we need to add to pt_regs the OLD_LR which I really don't like because it is > breaking the whole purpose of pt_regs (it should only contain one copy of each reg, > though it already has the ORIG_r0 in it) and will also break the stack alignment. >> >> > >> > -- >> > 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.
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