Re: [PATCH 6/6] arm64: switch to relative exception tables

From: Will Deacon
Date: Mon Jan 04 2016 - 09:46:51 EST

On Sun, Jan 03, 2016 at 05:05:57PM +0100, Ard Biesheuvel wrote:
> Instead of using absolute addresses for both the exception location
> and the fixup, use offsets relative to the exception table entry values.
> Not only does this cut the size of the exception table in half, it is
> also a prerequisite for KASLR, since absolute exception table entries
> are subject to dynamic relocation, which is incompatible with the sorting
> of the exception table that occurs at build time.
>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---
>
> Note that this patch supersedes the version I sent as part of the series that
> implements KASLR for arm64: http://thread.gmane.org/gmane.linux.kernel/2116531
>
> arch/arm64/include/asm/assembler.h | 2 +-
> arch/arm64/include/asm/futex.h | 4 ++--
> arch/arm64/include/asm/uaccess.h | 18 ++++++++++--------
> arch/arm64/kernel/armv8_deprecated.c | 4 ++--
> arch/arm64/mm/extable.c | 2 +-
> scripts/sortextable.c | 2 +-
> 6 files changed, 17 insertions(+), 15 deletions(-)

This looks good to me, so for the arm64 part (i.e. this patch):

Acked-by: Will Deacon <will.deacon@xxxxxxx>

Will

> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index 12eff928ef8b..8094d50f05bc 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -98,7 +98,7 @@
> 9999: x; \
> .section __ex_table,"a"; \
> .align 3; \
> - .quad 9999b,l; \
> + .long (9999b - .), (l - .); \
> .previous
>
> /*
> diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> index 007a69fc4f40..35e73e255ad3 100644
> --- a/arch/arm64/include/asm/futex.h
> +++ b/arch/arm64/include/asm/futex.h
> @@ -44,7 +44,7 @@
> " .popsection\n" \
> " .pushsection __ex_table,\"a\"\n" \
> " .align 3\n" \
> -" .quad 1b, 4b, 2b, 4b\n" \
> +" .long (1b - .), (4b - .), (2b - .), (4b - .)\n" \
> " .popsection\n" \
> ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN, \
> CONFIG_ARM64_PAN) \
> @@ -135,7 +135,7 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> " .popsection\n"
> " .pushsection __ex_table,\"a\"\n"
> " .align 3\n"
> -" .quad 1b, 4b, 2b, 4b\n"
> +" .long (1b - .), (4b - .), (2b - .), (4b - .)\n"
> " .popsection\n"
> : "+r" (ret), "=&r" (val), "+Q" (*uaddr), "=&r" (tmp)
> : "r" (oldval), "r" (newval), "Ir" (-EFAULT)
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index b2ede967fe7d..ab627e6c06c9 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -36,11 +36,11 @@
> #define VERIFY_WRITE 1
>
> /*
> - * The exception table consists of pairs of addresses: the first is the
> - * address of an instruction that is allowed to fault, and the second is
> - * the address at which the program should continue. No registers are
> - * modified, so it is entirely up to the continuation code to figure out
> - * what to do.
> + * The exception table consists of pairs of relative offsets: the first
> + * is the relative offset to an instruction that is allowed to fault,
> + * and the second is the relative offset at which the program should
> + * continue. No registers are modified, so it is entirely up to the
> + * continuation code to figure out what to do.
> *
> * All the routines below use bits of fixup code that are out of line
> * with the main instruction path. This means when everything is well,
> @@ -50,9 +50,11 @@
>
> struct exception_table_entry
> {
> - unsigned long insn, fixup;
> + int insn, fixup;
> };
>
> +#define ARCH_HAS_RELATIVE_EXTABLE
> +
> extern int fixup_exception(struct pt_regs *regs);
>
> #define KERNEL_DS (-1UL)
> @@ -125,7 +127,7 @@ static inline void set_fs(mm_segment_t fs)
> " .previous\n" \
> " .section __ex_table,\"a\"\n" \
> " .align 3\n" \
> - " .quad 1b, 3b\n" \
> + " .long (1b - .), (3b - .)\n" \
> " .previous" \
> : "+r" (err), "=&r" (x) \
> : "r" (addr), "i" (-EFAULT))
> @@ -192,7 +194,7 @@ do { \
> " .previous\n" \
> " .section __ex_table,\"a\"\n" \
> " .align 3\n" \
> - " .quad 1b, 3b\n" \
> + " .long (1b - .), (3b - .)\n" \
> " .previous" \
> : "+r" (err) \
> : "r" (x), "r" (addr), "i" (-EFAULT))
> diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
> index 937f5e58a4d3..8f21b1363387 100644
> --- a/arch/arm64/kernel/armv8_deprecated.c
> +++ b/arch/arm64/kernel/armv8_deprecated.c
> @@ -299,8 +299,8 @@ static void register_insn_emulation_sysctl(struct ctl_table *table)
> " .popsection" \
> " .pushsection __ex_table,\"a\"\n" \
> " .align 3\n" \
> - " .quad 0b, 4b\n" \
> - " .quad 1b, 4b\n" \
> + " .long (0b - .), (4b - .)\n" \
> + " .long (1b - .), (4b - .)\n" \
> " .popsection\n" \
> ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN, \
> CONFIG_ARM64_PAN) \
> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> index 79444279ba8c..81acd4706878 100644
> --- a/arch/arm64/mm/extable.c
> +++ b/arch/arm64/mm/extable.c
> @@ -11,7 +11,7 @@ int fixup_exception(struct pt_regs *regs)
>
> fixup = search_exception_tables(instruction_pointer(regs));
> if (fixup)
> - regs->pc = fixup->fixup;
> + regs->pc = (unsigned long)&fixup->fixup + fixup->fixup;
>
> return fixup != NULL;
> }
> diff --git a/scripts/sortextable.c b/scripts/sortextable.c
> index c2423d913b46..af247c70fb66 100644
> --- a/scripts/sortextable.c
> +++ b/scripts/sortextable.c
> @@ -282,12 +282,12 @@ do_file(char const *const fname)
> case EM_386:
> case EM_X86_64:
> case EM_S390:
> + case EM_AARCH64:
> custom_sort = sort_relative_table;
> break;
> case EM_ARCOMPACT:
> case EM_ARCV2:
> case EM_ARM:
> - case EM_AARCH64:
> case EM_MICROBLAZE:
> case EM_MIPS:
> case EM_XTENSA:
> --
> 1.9.1
>
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