Re: [PATCHv3 2/7] sections: split dereference_function_descriptor()
From: Petr Mladek <pmladek@suse.com>
Date: 2017-10-04 09:00:50
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On Sat 2017-09-30 11:53:14, Sergey Senozhatsky wrote:
There are two format specifiers to print out a pointer in symbolic format: '%pS/%ps' and '%pF/%pf'. On most architectures, the two mean exactly the same thing, but some architectures (ia64, ppc64, parisc64) use an indirect pointer for C function pointers, where the function pointer points to a function descriptor (which in turn contains the actual pointer to the code). The '%pF/%pf, when used appropriately, automatically does the appropriate function descriptor dereference on such architectures. The "when used appropriately" part is tricky. Basically this is a subtle ABI detail, specific to some platforms, that made it to the API level and people can be unaware of it and miss the whole "we need to dereference the function" business out. [1] proves that point (note that it fixes only '%pF' and '%pS', there might be '%pf' and '%ps' cases as well). It appears that we can handle everything within the affected arches and make '%pS/%ps' smart enough to retire '%pF/%pf'. Function descriptors live in .opd elf section and all affected arches (ia64, ppc64, parisc64) handle it properly for kernel and modules. So we, technically, can decide if the dereference is needed by simply looking at the pointer: if it belongs to .opd section then we need to dereference it.
Great catch. I really like this approach!
quoted hunk ↗ jump to hunk
diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h index e5da44eddd2f..387f22c41e0d 100644 --- a/include/asm-generic/sections.h +++ b/include/asm-generic/sections.h@@ -29,6 +29,7 @@ * __ctors_start, __ctors_end * __irqentry_text_start, __irqentry_text_end * __softirqentry_text_start, __softirqentry_text_end + * __start_opd, __end_opd */ extern char _text[], _stext[], _etext[]; extern char _data[], _sdata[], _edata[];@@ -47,12 +48,15 @@ extern char __softirqentry_text_start[], __softirqentry_text_end[]; /* Start and end of .ctors section - used for constructor calls. */ extern char __ctors_start[], __ctors_end[]; +/* Start and end of .opd section - used for function descriptors. */ +extern char __start_opd[], __end_opd[]; + extern __visible const void __nosave_begin, __nosave_end; -/* function descriptor handling (if any). Override - * in asm/sections.h */ +/* Function descriptor handling (if any). Override in asm/sections.h */ #ifndef dereference_function_descriptor #define dereference_function_descriptor(p) (p) +#define dereference_kernel_function_descriptor(p) (p) #endif /* random extra sections (if any). Overridediff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h index 4d0cb9bba93e..172904e9cded 100644 --- a/include/linux/moduleloader.h +++ b/include/linux/moduleloader.h@@ -85,6 +85,10 @@ void module_arch_cleanup(struct module *mod); /* Any cleanup before freeing mod->module_init */ void module_arch_freeing_init(struct module *mod); +/* Dereference module function descriptor */ +unsigned long dereference_module_function_descriptor(struct module *mod, + unsigned long addr); +
The function is used when the module is already loaded. IMHO, include/linux/module.h would be a better place. One advantage would be that we could use the same trick as in include/asm-generic/sections.h. I mean: #define dereference_module_function_descriptor(mod, addr) (addr) and redefine it in the three affected arch/<arch>/include/asm/module.h headers. Then it might be completely optimized out on all architectures. Anyway, we need to get ack from Jessica for this change. Best Regards, Petr