Messages in this thread Patch in this message | | | Date | Thu, 28 Apr 2022 13:15:22 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH 2/2] recordmcount: Handle sections with no non-weak symbols |
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Steven Rostedt wrote: > On Wed, 27 Apr 2022 15:01:22 +0530 > "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote: > >> If one or both of these weak functions are overridden in future, in the >> final vmlinux mcount table, references to these will change over to the >> non-weak variant which has its own mcount location entry. As such, there >> will now be two entries for these functions, both pointing to the same >> non-weak location. > > But is that really true in all cases? x86 uses fentry these days, and other > archs do things differently too. But the original mcount (-pg) call > happened *after* the frame setup. That means the offset of the mcount call > would be at different offsets wrt the start of the function. If you have > one of these architectures that still use mcount, and the weak function > doesn't have the same size frame setup as the overriding function, then the > addresses will not be the same.
Indeed, plain old -pg will be a problem. I'm not sure there is a generic way to address this. I suppose architectures will have to validate the mcount locations, something like this?
diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h index d83758acd1c7c3..d8b104ed2fdf38 100644 --- a/arch/powerpc/include/asm/ftrace.h +++ b/arch/powerpc/include/asm/ftrace.h @@ -12,13 +12,7 @@ #ifndef __ASSEMBLY__ extern void _mcount(void); - -static inline unsigned long ftrace_call_adjust(unsigned long addr) -{ - /* relocation of mcount call site is the same as the address */ - return addr; -} - +unsigned long ftrace_call_adjust(unsigned long addr); unsigned long prepare_ftrace_return(unsigned long parent, unsigned long ip, unsigned long sp); diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c index 4ee04aacf9f13c..976c08cd0573f7 100644 --- a/arch/powerpc/kernel/trace/ftrace.c +++ b/arch/powerpc/kernel/trace/ftrace.c @@ -858,6 +858,17 @@ void arch_ftrace_update_code(int command) ftrace_modify_all_code(command); } +unsigned long ftrace_call_adjust(unsigned long addr) +{ + ppc_inst_t op = ppc_inst_read((u32 *)addr); + + if (!is_bl_op(op)) + return 0; + + /* relocation of mcount call site is the same as the address */ + return addr; +} + #ifdef CONFIG_PPC64 #define PACATOC offsetof(struct paca_struct, kernel_toc) We can tighten those checks as necessary, but it will be upto the architectures to validate the mcount locations. This all will have to be opt-in so that only architectures doing necessary validation will allow mcount relocations against weak symbols.
- Naveen
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