Messages in this thread Patch in this message | | | Date | Sat, 9 Sep 2017 15:08:31 +0200 | From | Jessica Yu <> | Subject | Re: module: use relative references for __ksymtab entries |
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+++ Ard Biesheuvel [19/08/17 19:10 +0100]: >An ordinary arm64 defconfig build has ~64 KB worth of __ksymtab >entries, each consisting of two 64-bit fields containing absolute >references, to the symbol itself and to a char array containing >its name, respectively. > >When we build the same configuration with KASLR enabled, we end >up with an additional ~192 KB of relocations in the .init section, >i.e., one 24 byte entry for each absolute reference, which all need >to be processed at boot time. > >Given how the struct kernel_symbol that describes each entry is >completely local to module.c (except for the references emitted >by EXPORT_SYMBOL() itself), we can easily modify it to contain >two 32-bit relative references instead. This reduces the size of >the __ksymtab section by 50% for all 64-bit architectures, and >gets rid of the runtime relocations entirely for architectures >implementing KASLR, either via standard PIE linking (arm64) or >using custom host tools (x86). > >Note that the binary search involving __ksymtab contents relies >on each section being sorted by symbol name. This is implemented >based on the input section names, not the names in the ksymtab >entries, so this patch does not interfere with that. > >Given that the use of place-relative relocations requires support >both in the toolchain and in the module loader, we cannot enable >this feature for all architectures. So make it dependend on whether >CONFIG_HAVE_ARCH_PREL32_RELOCATIONS is defined. > >Cc: Jessica Yu <jeyu@kernel.org> >Cc: Arnd Bergmann <arnd@arndb.de> >Cc: Andrew Morton <akpm@linux-foundation.org> >Cc: Ingo Molnar <mingo@kernel.org> >Cc: Kees Cook <keescook@chromium.org> >Cc: Thomas Garnier <thgarnie@google.com> >Cc: Nicolas Pitre <nico@linaro.org> >Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
I ran this through some light testing and the relative ksymtab references seem to work just fine:
Acked-by: Jessica Yu <jeyu@kernel.org> (for module changes)
> arch/x86/include/asm/Kbuild | 1 + > arch/x86/include/asm/export.h | 4 -- > include/asm-generic/export.h | 12 +++++- > include/linux/compiler.h | 11 +++++ > include/linux/export.h | 45 +++++++++++++++----- > kernel/module.c | 33 +++++++++++--- > 6 files changed, 83 insertions(+), 23 deletions(-) > >diff --git a/arch/x86/include/asm/Kbuild b/arch/x86/include/asm/Kbuild >index 5d6a53fd7521..3e8a88dcaa1d 100644 >--- a/arch/x86/include/asm/Kbuild >+++ b/arch/x86/include/asm/Kbuild >@@ -9,5 +9,6 @@ generated-y += xen-hypercalls.h > generic-y += clkdev.h > generic-y += dma-contiguous.h > generic-y += early_ioremap.h >+generic-y += export.h > generic-y += mcs_spinlock.h > generic-y += mm-arch-hooks.h >diff --git a/arch/x86/include/asm/export.h b/arch/x86/include/asm/export.h >deleted file mode 100644 >index 138de56b13eb..000000000000 >--- a/arch/x86/include/asm/export.h >+++ /dev/null >@@ -1,4 +0,0 @@ >-#ifdef CONFIG_64BIT >-#define KSYM_ALIGN 16 >-#endif >-#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 eca8ad75e28b..363fb3e734ec 100644 >--- a/include/linux/compiler.h >+++ b/include/linux/compiler.h >@@ -590,4 +590,15 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s > (_________p1); \ > }) > >+/* >+ * 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 >+ * visibile to the compiler. >+ */ >+#define __ADDRESSABLE(sym) \ >+ static void *__attribute__((section(".discard.text"), used)) \ >+ __PASTE(__discard_##sym, __LINE__)(void) \ >+ { return (void *)&sym; } \ >+ > #endif /* __LINUX_COMPILER_H */ >diff --git a/include/linux/export.h b/include/linux/export.h >index 1a1dfdb2a5c6..156b974181a4 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 +54,46 @@ extern struct module __this_module; > #define __CRC_SYMBOL(sym, sec) > #endif > >+#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS >+/* >+ * 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 { >+ signed int value_offset; >+ signed 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(__KSYM_DEPS__) > >diff --git a/kernel/module.c b/kernel/module.c >index 40f983cbea81..a45423dcc32d 100644 >--- a/kernel/module.c >+++ b/kernel/module.c >@@ -539,12 +539,31 @@ 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)&sym->value_offset + 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 (const char *)((unsigned long)&sym->name_offset + >+ 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, >@@ -2190,7 +2209,7 @@ 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); > >@@ -2220,10 +2239,12 @@ 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; > } > } >@@ -2272,7 +2293,7 @@ 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; > } > >@@ -2532,7 +2553,7 @@ 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 */ >-- >2.11.0 >
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