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SubjectRe: [PATCH v8 3/6] module: use relative references for __ksymtab entries
On 25 June 2018 at 10:56, Martijn Coenen <maco@android.com> wrote:
> Hi Ard,
>
>> --- a/arch/x86/include/asm/export.h
>> +++ /dev/null
>> @@ -1,5 +0,0 @@
>> -/* SPDX-License-Identifier: GPL-2.0 */
>> -#ifdef CONFIG_64BIT
>> -#define KSYM_ALIGN 16
>> -#endif
>
> Why remove the 16-byte alignment here?

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 was working on some other
> code and ran into this 16-byte alignment, but I'm not sure why it's
> needed on x86_64 in the first place. Possibly because the ABI requires
> the stack to be 16-byte aligned when using sse instructions?
>

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.


>> -#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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