Messages in this thread Patch in this message | | | From | Sumit Garg <> | Date | Thu, 4 Aug 2022 14:48:41 +0530 | Subject | Re: [PATCH v3 0/2] arm64: Fix pending single-step debugging issues |
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On Mon, 11 Jul 2022 at 19:21, Sumit Garg <sumit.garg@linaro.org> wrote: > > On Mon, 11 Jul 2022 at 19:17, Doug Anderson <dianders@chromium.org> wrote: > > > > Hi, > > > > On Mon, Jul 11, 2022 at 5:44 AM Sumit Garg <sumit.garg@linaro.org> wrote: > > > > > > > I'll also note that I _think_ I remember that with Wei's series that > > > > the gdb function "call" started working. I tried that here and it > > > > didn't seem so happy. To keep things simple, I created a dummy > > > > function in my kernel that looked like: > > > > > > > > void doug_test(void) > > > > { > > > > pr_info("testing, 1 2 3\n"); > > > > } > > > > > > > > I broke into the debugger by echoing "g" to /proc/sysrq-trigger and > > > > then tried "call doug_test()". I guess my printout actually printed > > > > but it wasn't so happy after that. Seems like it somehow ended up > > > > returning to a bogus address after the call which then caused a crash. > > > > > > > > > > I am able to reproduce this issue on my setup as well. But it doesn't > > > seem to be a regression caused by this patch-set over Wei's series. As > > > I could reproduce this issue with v1 [1] patch-set as well which was > > > just a forward port of pending patches from Wei's series to the latest > > > upstream. > > > > > > Maybe it's a different regression caused by other changes? BTW, do you > > > remember the kernel version you tested with Wei's series applied? > > > > Sorry, I don't remember! :( I can't even be 100% sure that I'm > > remembering correctly that I tested it back in the day, so it's > > possible that it simply never worked... > > Okay, no worries. Let me see if I can come up with a separate fix for this. >
After digging deep into GDB call function operations for aarch64, it is certain that function calls simply never worked due to below reasons:
1. On aarch64, GDB call function inserts a breakpoint at the entrypoint of kernel (which is ffffffc008000000 from your dump) as return address from function called. And since it refers to the "_text" symbol which is marked non-executable as the actual text section starts with the "_stext" symbol. I did a following hack that makes it executable:
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index 626ec32873c6..e39ad1a5f5d6 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c @@ -700,7 +700,7 @@ static bool arm64_early_this_cpu_has_bti(void) static void __init map_kernel(pgd_t *pgdp) { static struct vm_struct vmlinux_text, vmlinux_rodata, vmlinux_inittext, - vmlinux_initdata, vmlinux_data; + vmlinux_initdata, vmlinux_data, vmlinux_htext;
/* * External debuggers may need to write directly to the text @@ -721,6 +721,8 @@ static void __init map_kernel(pgd_t *pgdp) * Only rodata will be remapped with different permissions later on, * all other segments are allowed to use contiguous mappings. */ + map_kernel_segment(pgdp, _text, _stext, text_prot, &vmlinux_htext, 0, + VM_NO_GUARD); map_kernel_segment(pgdp, _stext, _etext, text_prot, &vmlinux_text, 0, VM_NO_GUARD); map_kernel_segment(pgdp, __start_rodata, __inittext_begin, PAGE_KERNEL, 2. For the GDB function "call" to work, GDB inserts a dummy stack frame. But in case of kernel on aarch64, the stack used is common among the exception handler and the kernel threads. So it's not trivial to insert a dummy stack frame and requires rework of exception entry code as it pushes pt_regs onto the stack.
-Sumit
> > > > > -Doug
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