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    Subject[patch 1/2] x86 - Enhance DEBUG_RODATA support - alternatives
    Fix a memcpy that should be a text_poke (in apply_alternatives).

    Use kernel_wp_save/kernel_wp_restore in text_poke to support DEBUG_RODATA
    correctly and so the CPU HOTPLUG special case can be removed.

    Add text_poke_early, for alternatives and paravirt boot-time and module load
    time patching.

    Notes:
    - A macro is used instead of an inline function to deal with circular header
    include otherwise necessary for read_cr0 and preempt_disable/enable.

    Changelog:

    - Fix text_set and text_poke alignment check (mixed up bitwise and and or)
    - Remove text_set
    - Export add_nops, so it can be used by others.
    - Remove x86 test for "wp_works_ok", it will just be ignored by the architecture
    if not supported.
    - Document text_poke_early.
    - Remove clflush, since it breaks some VIA architectures and is not strictly
    necessary.
    - Add kerneldoc to text_poke and text_poke_early.
    - Remove arg cr0 from kernel_wp_save/restore. Change the macro name for
    kernel_wp_disable/enable.

    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    CC: Andi Kleen <andi@firstfloor.org>
    CC: pageexec@freemail.hu
    CC: Thomas Gleixner <tglx@linutronix.de>
    CC: Ingo Molnar <mingo@redhat.com>
    CC: H. Peter Anvin <hpa@zytor.com>
    ---
    arch/x86/kernel/alternative.c | 53 ++++++++++++++++++++++++++++++++++--------
    include/asm-x86/alternative.h | 36 +++++++++++++++++++++++++++-
    2 files changed, 79 insertions(+), 10 deletions(-)

    Index: linux-2.6-lttng/arch/x86/kernel/alternative.c
    ===================================================================
    --- linux-2.6-lttng.orig/arch/x86/kernel/alternative.c 2008-02-02 14:06:43.000000000 -0500
    +++ linux-2.6-lttng/arch/x86/kernel/alternative.c 2008-02-02 14:07:29.000000000 -0500
    @@ -173,7 +173,7 @@ static const unsigned char*const * find_
    #endif /* CONFIG_X86_64 */

    /* Use this to add nops to a buffer, then text_poke the whole buffer. */
    -static void add_nops(void *insns, unsigned int len)
    +void add_nops(void *insns, unsigned int len)
    {
    const unsigned char *const *noptable = find_nop_table();

    @@ -186,6 +186,7 @@ static void add_nops(void *insns, unsign
    len -= noplen;
    }
    }
    +EXPORT_SYMBOL_GPL(add_nops);

    extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
    extern u8 *__smp_locks[], *__smp_locks_end[];
    @@ -219,7 +220,7 @@ void apply_alternatives(struct alt_instr
    memcpy(insnbuf, a->replacement, a->replacementlen);
    add_nops(insnbuf + a->replacementlen,
    a->instrlen - a->replacementlen);
    - text_poke(instr, insnbuf, a->instrlen);
    + text_poke_early(instr, insnbuf, a->instrlen);
    }
    }

    @@ -407,7 +408,7 @@ void apply_paravirt(struct paravirt_patc

    /* Pad the rest with nops */
    add_nops(insnbuf + used, p->len - used);
    - text_poke(p->instr, insnbuf, p->len);
    + text_poke_early(p->instr, insnbuf, p->len);
    }
    }
    extern struct paravirt_patch_site __start_parainstructions[],
    @@ -471,18 +472,52 @@ void __init alternative_instructions(voi
    #endif
    }

    -/*
    - * Warning:
    +/**
    + * text_poke_early - Update instructions on a live kernel at boot time
    + * @addr: address to modify
    + * @opcode: source of the copy
    + * @len: length to copy
    + *
    * When you use this code to patch more than one byte of an instruction
    * you need to make sure that other CPUs cannot execute this code in parallel.
    - * Also no thread must be currently preempted in the middle of these instructions.
    - * And on the local CPU you need to be protected again NMI or MCE handlers
    - * seeing an inconsistent instruction while you patch.
    + * Also no thread must be currently preempted in the middle of these
    + * instructions. And on the local CPU you need to be protected again NMI or MCE
    + * handlers seeing an inconsistent instruction while you patch.
    + * Warning: read_cr0 is modified by paravirt, this is why we have _early
    + * versions. They are not in the __init section because they can be used at
    + * module load time.
    */
    -void __kprobes text_poke(void *addr, unsigned char *opcode, int len)
    +void *text_poke_early(void *addr, const void *opcode, size_t len)
    {
    memcpy(addr, opcode, len);
    sync_core();
    /* Could also do a CLFLUSH here to speed up CPU recovery; but
    that causes hangs on some VIA CPUs. */
    + return addr;
    }
    +
    +/**
    + * text_poke - Update instructions on a live kernel
    + * @addr: address to modify
    + * @opcode: source of the copy
    + * @len: length to copy
    + *
    + * Only atomic text poke/set should be allowed when not doing early patching.
    + * It means the size must be writable atomically and the address must be aligned
    + * in a way that permits an atomic write.
    + */
    +void *__kprobes text_poke(void *addr, const void *opcode, size_t len)
    +{
    + BUG_ON(len > sizeof(long));
    + BUG_ON((((long)addr + len - 1) & ~(sizeof(long) - 1))
    + - ((long)addr & ~(sizeof(long) - 1)));
    + kernel_wp_disable();
    + memcpy(addr, opcode, len);
    + kernel_wp_enable();
    + sync_core();
    + /* Could also do a CLFLUSH here to speed up CPU recovery; but
    + that causes hangs on some VIA CPUs. */
    + return addr;
    +}
    +
    +
    Index: linux-2.6-lttng/include/asm-x86/alternative.h
    ===================================================================
    --- linux-2.6-lttng.orig/include/asm-x86/alternative.h 2008-02-02 14:06:43.000000000 -0500
    +++ linux-2.6-lttng/include/asm-x86/alternative.h 2008-02-02 14:07:29.000000000 -0500
    @@ -4,6 +4,7 @@
    #include <linux/types.h>
    #include <linux/stddef.h>
    #include <asm/asm.h>
    +#include <asm/processor-flags.h>

    /*
    * Alternative inline assembly for SMP.
    @@ -156,6 +157,39 @@ apply_paravirt(struct paravirt_patch_sit
    #define __parainstructions_end NULL
    #endif

    -extern void text_poke(void *addr, unsigned char *opcode, int len);
    +extern void add_nops(void *insns, unsigned int len);
    +
    +/*
    + * Clear and restore the kernel write-protection flag on the local CPU.
    + * Allows the kernel to edit read-only pages.
    + * Side-effect: any interrupt handler running between save and restore will have
    + * the ability to write to read-only pages.
    + *
    + * Warning:
    + * Code patching in the UP case is safe if NMIs and MCE handlers are stopped and
    + * no thread can be preempted in the instructions being modified (no iret to an
    + * invalid instruction possible) or if the instructions are changed from a
    + * consistent state to another consistent state atomically.
    + * More care must be taken when modifying code in the SMP case because of
    + * Intel's errata.
    + * On the local CPU you need to be protected again NMI or MCE handlers seeing an
    + * inconsistent instruction while you patch.
    + * The _early version does not use read_cr0(), which can be paravirtualized.
    + */
    +
    +extern void *text_poke(void *addr, const void *opcode, size_t len);
    +extern void *text_poke_early(void *addr, const void *opcode, size_t len);
    +
    +#define kernel_wp_disable() \
    +do { \
    + preempt_disable(); \
    + write_cr0(read_cr0() & ~X86_CR0_WP); \
    +} while (0)
    +
    +#define kernel_wp_enable() \
    +do { \
    + write_cr0(read_cr0() | X86_CR0_WP); \
    + preempt_enable(); \
    +} while (0)

    #endif /* _ASM_X86_ALTERNATIVE_H */
    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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