On Mon, May 02, 2016 at 08:52:41AM -0700, Andy Lutomirski wrote: > On Mon, May 2, 2016 at 6:52 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > On Fri, Apr 29, 2016 at 05:08:50PM -0700, Andy Lutomirski wrote: > >> On Apr 29, 2016 3:41 PM, "Josh Poimboeuf" <jpoimboe@xxxxxxxxxx> wrote: > >> > > >> > On Fri, Apr 29, 2016 at 02:37:41PM -0700, Andy Lutomirski wrote: > >> > > On Fri, Apr 29, 2016 at 2:25 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > >> > > >> I suppose we could try to rejigger the code so that rbp points to > >> > > >> pt_regs or similar. > >> > > > > >> > > > I think we should avoid doing something like that because it would break > >> > > > gdb and all the other unwinders who don't know about it. > >> > > > >> > > How so? > >> > > > >> > > Currently, rbp in the entry code is meaningless. I'm suggesting that, > >> > > when we do, for example, 'call \do_sym' in idtentry, we point rbp to > >> > > the pt_regs. Currently it points to something stale (which the > >> > > dump_stack code might be relying on. Hmm.) But it's probably also > >> > > safe to assume that if you unwind to the 'call \do_sym', then pt_regs > >> > > is the next thing on the stack, so just doing the section thing would > >> > > work. > >> > > >> > Yes, rbp is meaningless on the entry from user space. But if an > >> > in-kernel interrupt occurs (e.g. page fault, preemption) and you have > >> > nested entry, rbp keeps its old value, right? So the unwinder can walk > >> > past the nested entry frame and keep going until it gets to the original > >> > entry. > >> > >> Yes. > >> > >> It would be nice if we could do better, though, and actually notice > >> the pt_regs and identify the entry. For example, I'd love to see > >> "page fault, RIP=xyz" printed in the middle of a stack dump on a > >> crash. > >> > >> Also, I think that just following rbp links will lose the > >> actual function that took the page fault (or whatever function > >> pt_regs->ip actually points to). > > > > Hm. I think we could fix all that in a more standard way. Whenever a > > new pt_regs frame gets saved on entry, we could also create a new stack > > frame which points to a fake kernel_entry() function. That would tell > > the unwinder there's a pt_regs frame without otherwise breaking frame > > pointers across the frame. > > > > Then I guess we wouldn't need my other solution of putting the idt > > entries in a special section. > > > > How does that sound? > > Let me try to understand. > > The normal call sequence is call; push %rbp; mov %rsp, %rbp. So rbp > points to (prev rbp, prev rip) on the stack, and you can follow the > chain back. Right now, on a user access page fault or similar, we > have rbp (probably) pointing to the interrupted frame, and the > interrupted rip isn't saved anywhere that a naive unwinder can find > it. (It's in pt_regs, but the rbp chain skips right over that.) > > We could change the entry code so that an interrupt / idtentry does: > > push pt_regs > push kernel_entry > push %rbp > mov %rsp, %rbp > call handler > pop %rbp > addq $8, %rsp > > or similar. That would make it appear that the actual C handler was > caused by a dummy function "kernel_entry". Now the unwinder would get > to kernel_entry, but it *still* wouldn't find its way to the calling > frame, which only solves part of the problem. We could at least teach > the unwinder how kernel_entry works and let it decode pt_regs to > continue unwinding. This would be nice, and I think it could work. > > I think I like this, except that, if it used a separate section, it > could potentially be faster, as, for each actual entry type, the > offset from the C handler frame to pt_regs is a foregone conclusion. > But this is pretty simple and performance is already abysmal in most > handlers. > > There's an added benefit to using a separate section, though: we could > also annotate the calls with what type of entry they were so the > unwinder could print it out nicely. > > I could be convinced either way. Ok, I took a stab at this. See the patch below. In addition to annotating interrupt/exception pt_regs frames, I also annotated all the syscall pt_regs, for consistency. As you mentioned, it will affect performance a bit, but I think it will be insignificant. I think I like this approach better than putting the interrupt/idtentry's in a special section, because this is much more precise. Especially now that I'm annotating pt_regs syscalls. Also I think with a few minor changes we could implement your idea of annotating the calls with what type of entry they are. But I don't think that's really needed, because the name of the interrupt/idtentry is already on the stack trace. Before: [<ffffffff8143c243>] dump_stack+0x85/0xc2 [<ffffffff81073596>] __do_page_fault+0x576/0x5a0 [<ffffffff8107369c>] trace_do_page_fault+0x5c/0x2e0 [<ffffffff8106d83c>] do_async_page_fault+0x2c/0xa0 [<ffffffff81887058>] async_page_fault+0x28/0x30 [<ffffffff81451560>] ? copy_page_to_iter+0x70/0x440 [<ffffffff811ebeac>] ? pagecache_get_page+0x2c/0x290 [<ffffffff811edaeb>] generic_file_read_iter+0x26b/0x770 [<ffffffff81285e32>] __vfs_read+0xe2/0x140 [<ffffffff81286378>] vfs_read+0x98/0x140 [<ffffffff812878c8>] SyS_read+0x58/0xc0 [<ffffffff81884dbc>] entry_SYSCALL_64_fastpath+0x1f/0xbd After: [<ffffffff8143c243>] dump_stack+0x85/0xc2 [<ffffffff81073596>] __do_page_fault+0x576/0x5a0 [<ffffffff8107369c>] trace_do_page_fault+0x5c/0x2e0 [<ffffffff8106d83c>] do_async_page_fault+0x2c/0xa0 [<ffffffff81887422>] async_page_fault+0x32/0x40 [<ffffffff81887861>] pt_regs+0x1/0x10 [<ffffffff81451560>] ? copy_page_to_iter+0x70/0x440 [<ffffffff811ebeac>] ? pagecache_get_page+0x2c/0x290 [<ffffffff811edaeb>] generic_file_read_iter+0x26b/0x770 [<ffffffff81285e32>] __vfs_read+0xe2/0x140 [<ffffffff81286378>] vfs_read+0x98/0x140 [<ffffffff812878c8>] SyS_read+0x58/0xc0 [<ffffffff81884dc6>] entry_SYSCALL_64_fastpath+0x29/0xdb [<ffffffff81887861>] pt_regs+0x1/0x10 Note this example is with today's unwinder. It could be made smarter to get the RIP from the pt_regs so the '?' could be removed from copy_page_to_iter(). Thoughts? diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h index 9a9e588..f54886a 100644 --- a/arch/x86/entry/calling.h +++ b/arch/x86/entry/calling.h @@ -201,6 +201,32 @@ For 32-bit we have the following conventions - kernel is built with .byte 0xf1 .endm + /* + * Create a stack frame for the saved pt_regs. This allows frame + * pointer based unwinders to find pt_regs on the stack. + */ + .macro CREATE_PT_REGS_FRAME regs=%rsp +#ifdef CONFIG_FRAME_POINTER + pushq \regs + pushq $pt_regs+1 + pushq %rbp + movq %rsp, %rbp +#endif + .endm + + .macro REMOVE_PT_REGS_FRAME +#ifdef CONFIG_FRAME_POINTER + popq %rbp + addq $0x10, %rsp +#endif + .endm + + .macro CALL_HANDLER handler regs=%rsp + CREATE_PT_REGS_FRAME \regs + call \handler + REMOVE_PT_REGS_FRAME + .endm + #endif /* CONFIG_X86_64 */ /* diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 9ee0da1..8642984 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -199,6 +199,7 @@ entry_SYSCALL_64_fastpath: ja 1f /* return -ENOSYS (already in pt_regs->ax) */ movq %r10, %rcx + CREATE_PT_REGS_FRAME /* * This call instruction is handled specially in stub_ptregs_64. * It might end up jumping to the slow path. If it jumps, RAX @@ -207,6 +208,8 @@ entry_SYSCALL_64_fastpath: call *sys_call_table(, %rax, 8) .Lentry_SYSCALL_64_after_fastpath_call: + REMOVE_PT_REGS_FRAME + movq %rax, RAX(%rsp) 1: @@ -238,14 +241,14 @@ entry_SYSCALL_64_fastpath: ENABLE_INTERRUPTS(CLBR_NONE) SAVE_EXTRA_REGS movq %rsp, %rdi - call syscall_return_slowpath /* returns with IRQs disabled */ + CALL_HANDLER syscall_return_slowpath /* returns with IRQs disabled */ jmp return_from_SYSCALL_64 entry_SYSCALL64_slow_path: /* IRQs are off. */ SAVE_EXTRA_REGS movq %rsp, %rdi - call do_syscall_64 /* returns with IRQs disabled */ + CALL_HANDLER do_syscall_64 /* returns with IRQs disabled */ return_from_SYSCALL_64: RESTORE_EXTRA_REGS @@ -344,6 +347,7 @@ ENTRY(stub_ptregs_64) DISABLE_INTERRUPTS(CLBR_NONE) TRACE_IRQS_OFF popq %rax + REMOVE_PT_REGS_FRAME jmp entry_SYSCALL64_slow_path 1: @@ -372,7 +376,7 @@ END(ptregs_\func) ENTRY(ret_from_fork) LOCK ; btr $TIF_FORK, TI_flags(%r8) - call schedule_tail /* rdi: 'prev' task parameter */ + CALL_HANDLER schedule_tail /* rdi: 'prev' task parameter */ testb $3, CS(%rsp) /* from kernel_thread? */ jnz 1f @@ -385,8 +389,9 @@ ENTRY(ret_from_fork) * parameter to be passed in RBP. The called function is permitted * to call do_execve and thereby jump to user mode. */ + movq RBX(%rsp), %rbx movq RBP(%rsp), %rdi - call *RBX(%rsp) + CALL_HANDLER *%rbx movl $0, RAX(%rsp) /* @@ -396,7 +401,7 @@ ENTRY(ret_from_fork) 1: movq %rsp, %rdi - call syscall_return_slowpath /* returns with IRQs disabled */ + CALL_HANDLER syscall_return_slowpath /* returns with IRQs disabled */ TRACE_IRQS_ON /* user mode is traced as IRQS on */ SWAPGS jmp restore_regs_and_iret @@ -468,7 +473,7 @@ END(irq_entries_start) /* We entered an interrupt context - irqs are off: */ TRACE_IRQS_OFF - call \func /* rdi points to pt_regs */ + CALL_HANDLER \func regs=%rdi .endm /* @@ -495,7 +500,7 @@ ret_from_intr: /* Interrupt came from user space */ GLOBAL(retint_user) mov %rsp,%rdi - call prepare_exit_to_usermode + CALL_HANDLER prepare_exit_to_usermode TRACE_IRQS_IRETQ SWAPGS jmp restore_regs_and_iret @@ -509,7 +514,7 @@ retint_kernel: jnc 1f 0: cmpl $0, PER_CPU_VAR(__preempt_count) jnz 1f - call preempt_schedule_irq + CALL_HANDLER preempt_schedule_irq jmp 0b 1: #endif @@ -688,8 +693,6 @@ ENTRY(\sym) .endif .endif - movq %rsp, %rdi /* pt_regs pointer */ - .if \has_error_code movq ORIG_RAX(%rsp), %rsi /* get error code */ movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */ @@ -701,7 +704,8 @@ ENTRY(\sym) subq $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist) .endif - call \do_sym + movq %rsp, %rdi /* pt_regs pointer */ + CALL_HANDLER \do_sym .if \shift_ist != -1 addq $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist) @@ -728,8 +732,6 @@ ENTRY(\sym) call sync_regs movq %rax, %rsp /* switch stack */ - movq %rsp, %rdi /* pt_regs pointer */ - .if \has_error_code movq ORIG_RAX(%rsp), %rsi /* get error code */ movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */ @@ -737,7 +739,8 @@ ENTRY(\sym) xorl %esi, %esi /* no error code */ .endif - call \do_sym + movq %rsp, %rdi /* pt_regs pointer */ + CALL_HANDLER \do_sym jmp error_exit /* %ebx: no swapgs flag */ .endif @@ -1174,7 +1177,7 @@ ENTRY(nmi) movq %rsp, %rdi movq $-1, %rsi - call do_nmi + CALL_HANDLER do_nmi /* * Return back to user mode. We must *not* do the normal exit @@ -1387,7 +1390,7 @@ end_repeat_nmi: /* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */ movq %rsp, %rdi movq $-1, %rsi - call do_nmi + CALL_HANDLER do_nmi testl %ebx, %ebx /* swapgs needed? */ jnz nmi_restore @@ -1423,3 +1426,11 @@ ENTRY(ignore_sysret) mov $-ENOSYS, %eax sysret END(ignore_sysret) + +/* fake function which allows stack unwinders to detect pt_regs frames */ +#ifdef CONFIG_FRAME_POINTER +ENTRY(pt_regs) + nop + nop +ENDPROC(pt_regs) +#endif /* CONFIG_FRAME_POINTER */ -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
