Re: [PATCH v8 3/6] module: use relative references for __ksymtab entries

From: Ard Biesheuvel
Date: Mon Jun 25 2018 - 06:52:54 EST

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On 25 June 2018 at 12:15, Martijn Coenen <maco@xxxxxxxxxxx> wrote:
> On Mon, Jun 25, 2018 at 11:14 AM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> Because struct kernel_symbol is only 8 bytes in size after this
>> change, and aligning a 8 byte quantity to 16 bytes wastes 8 bytes.
>
> I get that, but then that means the 16-byte alignment wasn't actually
> necessary in the first place.
>

Not really, I suppose, although it is not unusual to align large
arrays to the entry size if it is a power of 2, so they don't span
cachelines.

>> The x86 ABI may require it, but we don't actually adhere to it in the
>> kernel. Also, these structures never occur on the stack anyway.
>
> Ok, makes sense.
>
> Thanks,
> Martijn
>
>>
>>
>>>> -#include <asm-generic/export.h>
>>>> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
>>>> index 719db1968d81..97ce606459ae 100644
>>>> --- a/include/asm-generic/export.h
>>>> +++ b/include/asm-generic/export.h
>>>> @@ -5,12 +5,10 @@
>>>> #define KSYM_FUNC(x) x
>>>> #endif
>>>> #ifdef CONFIG_64BIT
>>>> -#define __put .quad
>>>> #ifndef KSYM_ALIGN
>>>> #define KSYM_ALIGN 8
>>>> #endif
>>>> #else
>>>> -#define __put .long
>>>> #ifndef KSYM_ALIGN
>>>> #define KSYM_ALIGN 4
>>>> #endif
>>>> @@ -25,6 +23,16 @@
>>>> #define KSYM(name) name
>>>> #endif
>>>>
>>>> +.macro __put, val, name
>>>> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
>>>> + .long \val - ., \name - .
>>>> +#elif defined(CONFIG_64BIT)
>>>> + .quad \val, \name
>>>> +#else
>>>> + .long \val, \name
>>>> +#endif
>>>> +.endm
>>>> +
>>>> /*
>>>> * note on .section use: @progbits vs %progbits nastiness doesn't matter,
>>>> * since we immediately emit into those sections anyway.
>>>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>>>> index ab4711c63601..0a9328ea9dbd 100644
>>>> --- a/include/linux/compiler.h
>>>> +++ b/include/linux/compiler.h
>>>> @@ -280,6 +280,25 @@ unsigned long read_word_at_a_time(const void *addr)
>>>>
>>>> #endif /* __KERNEL__ */
>>>>
>>>> +/*
>>>> + * Force the compiler to emit 'sym' as a symbol, so that we can reference
>>>> + * it from inline assembler. Necessary in case 'sym' could be inlined
>>>> + * otherwise, or eliminated entirely due to lack of references that are
>>>> + * visible to the compiler.
>>>> + */
>>>> +#define __ADDRESSABLE(sym) \
>>>> + static void * const __attribute__((section(".discard"), used)) \
>>>> + __PASTE(__addressable_##sym, __LINE__) = (void *)&sym;
>>>> +
>>>> +/**
>>>> + * offset_to_ptr - convert a relative memory offset to an absolute pointer
>>>> + * @off: the address of the 32-bit offset value
>>>> + */
>>>> +static inline void *offset_to_ptr(const int *off)
>>>> +{
>>>> + return (void *)((unsigned long)off + *off);
>>>> +}
>>>> +
>>>> #endif /* __ASSEMBLY__ */
>>>>
>>>> #ifndef __optimize
>>>> diff --git a/include/linux/export.h b/include/linux/export.h
>>>> index 25005b55b079..04c78e6bfec9 100644
>>>> --- a/include/linux/export.h
>>>> +++ b/include/linux/export.h
>>>> @@ -24,12 +24,6 @@
>>>> #define VMLINUX_SYMBOL_STR(x) __VMLINUX_SYMBOL_STR(x)
>>>>
>>>> #ifndef __ASSEMBLY__
>>>> -struct kernel_symbol
>>>> -{
>>>> - unsigned long value;
>>>> - const char *name;
>>>> -};
>>>> -
>>>> #ifdef MODULE
>>>> extern struct module __this_module;
>>>> #define THIS_MODULE (&__this_module)
>>>> @@ -60,17 +54,47 @@ extern struct module __this_module;
>>>> #define __CRC_SYMBOL(sym, sec)
>>>> #endif
>>>>
>>>> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
>>>> +#include <linux/compiler.h>
>>>> +/*
>>>> + * Emit the ksymtab entry as a pair of relative references: this reduces
>>>> + * the size by half on 64-bit architectures, and eliminates the need for
>>>> + * absolute relocations that require runtime processing on relocatable
>>>> + * kernels.
>>>> + */
>>>> +#define __KSYMTAB_ENTRY(sym, sec) \
>>>> + __ADDRESSABLE(sym) \
>>>> + asm(" .section \"___ksymtab" sec "+" #sym "\", \"a\" \n" \
>>>> + " .balign 8 \n" \
>>>> + VMLINUX_SYMBOL_STR(__ksymtab_##sym) ": \n" \
>>>> + " .long " VMLINUX_SYMBOL_STR(sym) "- . \n" \
>>>> + " .long " VMLINUX_SYMBOL_STR(__kstrtab_##sym) "- .\n" \
>>>> + " .previous \n")
>>>> +
>>>> +struct kernel_symbol {
>>>> + int value_offset;
>>>> + int name_offset;
>>>> +};
>>>> +#else
>>>> +#define __KSYMTAB_ENTRY(sym, sec) \
>>>> + static const struct kernel_symbol __ksymtab_##sym \
>>>> + __attribute__((section("___ksymtab" sec "+" #sym), used)) \
>>>> + = { (unsigned long)&sym, __kstrtab_##sym }
>>>> +
>>>> +struct kernel_symbol {
>>>> + unsigned long value;
>>>> + const char *name;
>>>> +};
>>>> +#endif
>>>> +
>>>> /* For every exported symbol, place a struct in the __ksymtab section */
>>>> #define ___EXPORT_SYMBOL(sym, sec) \
>>>> extern typeof(sym) sym; \
>>>> __CRC_SYMBOL(sym, sec) \
>>>> static const char __kstrtab_##sym[] \
>>>> - __attribute__((section("__ksymtab_strings"), aligned(1))) \
>>>> + __attribute__((section("__ksymtab_strings"), used, aligned(1))) \
>>>> = VMLINUX_SYMBOL_STR(sym); \
>>>> - static const struct kernel_symbol __ksymtab_##sym \
>>>> - __used \
>>>> - __attribute__((section("___ksymtab" sec "+" #sym), used)) \
>>>> - = { (unsigned long)&sym, __kstrtab_##sym }
>>>> + __KSYMTAB_ENTRY(sym, sec)
>>>>
>>>> #if defined(__DISABLE_EXPORTS)
>>>>
>>>> diff --git a/kernel/module.c b/kernel/module.c
>>>> index ad2d420024f6..b4782cfbb79b 100644
>>>> --- a/kernel/module.c
>>>> +++ b/kernel/module.c
>>>> @@ -549,12 +549,30 @@ static bool check_symbol(const struct symsearch *syms,
>>>> return true;
>>>> }
>>>>
>>>> +static unsigned long kernel_symbol_value(const struct kernel_symbol *sym)
>>>> +{
>>>> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
>>>> + return (unsigned long)offset_to_ptr(&sym->value_offset);
>>>> +#else
>>>> + return sym->value;
>>>> +#endif
>>>> +}
>>>> +
>>>> +static const char *kernel_symbol_name(const struct kernel_symbol *sym)
>>>> +{
>>>> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
>>>> + return offset_to_ptr(&sym->name_offset);
>>>> +#else
>>>> + return sym->name;
>>>> +#endif
>>>> +}
>>>> +
>>>> static int cmp_name(const void *va, const void *vb)
>>>> {
>>>> const char *a;
>>>> const struct kernel_symbol *b;
>>>> a = va; b = vb;
>>>> - return strcmp(a, b->name);
>>>> + return strcmp(a, kernel_symbol_name(b));
>>>> }
>>>>
>>>> static bool find_symbol_in_section(const struct symsearch *syms,
>>>> @@ -2198,7 +2216,7 @@ void *__symbol_get(const char *symbol)
>>>> sym = NULL;
>>>> preempt_enable();
>>>>
>>>> - return sym ? (void *)sym->value : NULL;
>>>> + return sym ? (void *)kernel_symbol_value(sym) : NULL;
>>>> }
>>>> EXPORT_SYMBOL_GPL(__symbol_get);
>>>>
>>>> @@ -2228,10 +2246,12 @@ static int verify_export_symbols(struct module *mod)
>>>>
>>>> for (i = 0; i < ARRAY_SIZE(arr); i++) {
>>>> for (s = arr[i].sym; s < arr[i].sym + arr[i].num; s++) {
>>>> - if (find_symbol(s->name, &owner, NULL, true, false)) {
>>>> + if (find_symbol(kernel_symbol_name(s), &owner, NULL,
>>>> + true, false)) {
>>>> pr_err("%s: exports duplicate symbol %s"
>>>> " (owned by %s)\n",
>>>> - mod->name, s->name, module_name(owner));
>>>> + mod->name, kernel_symbol_name(s),
>>>> + module_name(owner));
>>>> return -ENOEXEC;
>>>> }
>>>> }
>>>> @@ -2280,7 +2300,7 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
>>>> ksym = resolve_symbol_wait(mod, info, name);
>>>> /* Ok if resolved. */
>>>> if (ksym && !IS_ERR(ksym)) {
>>>> - sym[i].st_value = ksym->value;
>>>> + sym[i].st_value = kernel_symbol_value(ksym);
>>>> break;
>>>> }
>>>>
>>>> @@ -2540,7 +2560,7 @@ static int is_exported(const char *name, unsigned long value,
>>>> ks = lookup_symbol(name, __start___ksymtab, __stop___ksymtab);
>>>> else
>>>> ks = lookup_symbol(name, mod->syms, mod->syms + mod->num_syms);
>>>> - return ks != NULL && ks->value == value;
>>>> + return ks != NULL && kernel_symbol_value(ks) == value;
>>>> }
>>>>
>>>> /* As per nm */
>>>> --
>>>> 2.15.1
>>>>

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In reply to: Martijn Coenen: "Re: [PATCH v8 3/6] module: use relative references for __ksymtab entries"
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