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    SubjectRe: [PATCH v2 08/13] asm-generic: Refactor dereference_[kernel]_function_descriptor()
    Excerpts from Christophe Leroy's message of October 14, 2021 3:49 pm:
    > dereference_function_descriptor() and
    > dereference_kernel_function_descriptor() are identical on the
    > three architectures implementing them.
    >
    > Make them common and put them out-of-line in kernel/extable.c
    > which is one of the users and has similar type of functions.

    We should be moving more stuff out of extable.c (including all the
    kernel address tests). lib/kimage.c or kelf.c or something.

    It could be after your series though.

    >
    > Reviewed-by: Kees Cook <keescook@chromium.org>
    > Reviewed-by: Arnd Bergmann <arnd@arndb.de>
    > Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
    > ---
    > arch/ia64/include/asm/sections.h | 19 -------------------
    > arch/parisc/include/asm/sections.h | 9 ---------
    > arch/parisc/kernel/process.c | 21 ---------------------
    > arch/powerpc/include/asm/sections.h | 23 -----------------------
    > include/asm-generic/sections.h | 2 ++
    > kernel/extable.c | 23 ++++++++++++++++++++++-
    > 6 files changed, 24 insertions(+), 73 deletions(-)
    >
    > diff --git a/arch/ia64/include/asm/sections.h b/arch/ia64/include/asm/sections.h
    > index 1aaed8882294..96c9bb500c34 100644
    > --- a/arch/ia64/include/asm/sections.h
    > +++ b/arch/ia64/include/asm/sections.h
    > @@ -31,23 +31,4 @@ extern char __start_gate_brl_fsys_bubble_down_patchlist[], __end_gate_brl_fsys_b
    > extern char __start_unwind[], __end_unwind[];
    > extern char __start_ivt_text[], __end_ivt_text[];
    >
    > -#undef dereference_function_descriptor
    > -static inline void *dereference_function_descriptor(void *ptr)
    > -{
    > - struct fdesc *desc = ptr;
    > - void *p;
    > -
    > - if (!get_kernel_nofault(p, (void *)&desc->addr))
    > - ptr = p;
    > - return ptr;
    > -}
    > -
    > -#undef dereference_kernel_function_descriptor
    > -static inline void *dereference_kernel_function_descriptor(void *ptr)
    > -{
    > - if (ptr < (void *)__start_opd || ptr >= (void *)__end_opd)
    > - return ptr;
    > - return dereference_function_descriptor(ptr);
    > -}
    > -
    > #endif /* _ASM_IA64_SECTIONS_H */
    > diff --git a/arch/parisc/include/asm/sections.h b/arch/parisc/include/asm/sections.h
    > index 37b34b357cb5..6b1fe22baaf5 100644
    > --- a/arch/parisc/include/asm/sections.h
    > +++ b/arch/parisc/include/asm/sections.h
    > @@ -13,13 +13,4 @@ typedef Elf64_Fdesc func_desc_t;
    >
    > extern char __alt_instructions[], __alt_instructions_end[];
    >
    > -#ifdef CONFIG_64BIT
    > -
    > -#undef dereference_function_descriptor
    > -void *dereference_function_descriptor(void *);
    > -
    > -#undef dereference_kernel_function_descriptor
    > -void *dereference_kernel_function_descriptor(void *);
    > -#endif
    > -
    > #endif
    > diff --git a/arch/parisc/kernel/process.c b/arch/parisc/kernel/process.c
    > index 38ec4ae81239..7382576b52a8 100644
    > --- a/arch/parisc/kernel/process.c
    > +++ b/arch/parisc/kernel/process.c
    > @@ -266,27 +266,6 @@ get_wchan(struct task_struct *p)
    > return 0;
    > }
    >
    > -#ifdef CONFIG_64BIT
    > -void *dereference_function_descriptor(void *ptr)
    > -{
    > - Elf64_Fdesc *desc = ptr;
    > - void *p;
    > -
    > - if (!get_kernel_nofault(p, (void *)&desc->addr))
    > - ptr = p;
    > - return ptr;
    > -}
    > -
    > -void *dereference_kernel_function_descriptor(void *ptr)
    > -{
    > - if (ptr < (void *)__start_opd ||
    > - ptr >= (void *)__end_opd)
    > - return ptr;
    > -
    > - return dereference_function_descriptor(ptr);
    > -}
    > -#endif
    > -
    > static inline unsigned long brk_rnd(void)
    > {
    > return (get_random_int() & BRK_RND_MASK) << PAGE_SHIFT;
    > diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
    > index 1322d7b2f1a3..fbfe1957edbe 100644
    > --- a/arch/powerpc/include/asm/sections.h
    > +++ b/arch/powerpc/include/asm/sections.h
    > @@ -72,29 +72,6 @@ static inline int overlaps_kernel_text(unsigned long start, unsigned long end)
    > (unsigned long)_stext < end;
    > }
    >
    > -#ifdef PPC64_ELF_ABI_v1
    > -
    > -#undef dereference_function_descriptor
    > -static inline void *dereference_function_descriptor(void *ptr)
    > -{
    > - struct ppc64_opd_entry *desc = ptr;
    > - void *p;
    > -
    > - if (!get_kernel_nofault(p, (void *)&desc->addr))
    > - ptr = p;
    > - return ptr;
    > -}
    > -
    > -#undef dereference_kernel_function_descriptor
    > -static inline void *dereference_kernel_function_descriptor(void *ptr)
    > -{
    > - if (ptr < (void *)__start_opd || ptr >= (void *)__end_opd)
    > - return ptr;
    > -
    > - return dereference_function_descriptor(ptr);
    > -}
    > -#endif /* PPC64_ELF_ABI_v1 */
    > -
    > #endif
    >
    > #endif /* __KERNEL__ */
    > diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
    > index cbec7d5f1678..76163883c6ff 100644
    > --- a/include/asm-generic/sections.h
    > +++ b/include/asm-generic/sections.h
    > @@ -60,6 +60,8 @@ extern __visible const void __nosave_begin, __nosave_end;
    >
    > /* Function descriptor handling (if any). Override in asm/sections.h */
    > #ifdef HAVE_FUNCTION_DESCRIPTORS
    > +void *dereference_function_descriptor(void *ptr);
    > +void *dereference_kernel_function_descriptor(void *ptr);
    > #else
    > #define dereference_function_descriptor(p) ((void *)(p))
    > #define dereference_kernel_function_descriptor(p) ((void *)(p))
    > diff --git a/kernel/extable.c b/kernel/extable.c
    > index b0ea5eb0c3b4..013ccffade11 100644
    > --- a/kernel/extable.c
    > +++ b/kernel/extable.c
    > @@ -3,6 +3,7 @@
    > Copyright (C) 2001 Rusty Russell, 2002 Rusty Russell IBM.
    >
    > */
    > +#include <linux/elf.h>
    > #include <linux/ftrace.h>
    > #include <linux/memory.h>
    > #include <linux/extable.h>
    > @@ -159,12 +160,32 @@ int kernel_text_address(unsigned long addr)
    > }
    >
    > /*
    > - * On some architectures (PPC64, IA64) function pointers
    > + * On some architectures (PPC64, IA64, PARISC) function pointers
    > * are actually only tokens to some data that then holds the
    > * real function address. As a result, to find if a function
    > * pointer is part of the kernel text, we need to do some
    > * special dereferencing first.
    > */
    > +#ifdef HAVE_FUNCTION_DESCRIPTORS
    > +void *dereference_function_descriptor(void *ptr)
    > +{
    > + func_desc_t *desc = ptr;
    > + void *p;
    > +
    > + if (!get_kernel_nofault(p, (void *)&desc->addr))
    > + ptr = p;

    I know you're just copying existing code. This seems a bit risky though.
    I don't think anything good could come of just treating the descriptor
    address like a function entry address if we failed to load from it for
    whatever reason.

    Existing callers might be benign but the API is not good. It should
    give a nice fail return or BUG. If we change that then we should also
    change the name and pass the correct type to it too.

    Thanks,
    Nick

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