Re: [PATCH] ARM: Use generic BUG() handler

From: Simon Glass
Date: Tue Mar 29 2011 - 15:02:27 EST


Hi, does anyone have any more comments on this? Has anyone tried it
apart from anish kumar? Thanks, Simon

On Wed, Mar 16, 2011 at 1:27 PM, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
> From: Simon Glass <sjg@xxxxxxxxxx>
>
> ARM uses its own BUG() handler which makes its output slightly different
> from other archtectures.
>
> One of the problems is that the ARM implementation doesn't report the function
> with the BUG() in it, but always reports the PC being in __bug(). The generic
> implementation doesn't have this problem.
>
> Currently we get something like:
>
> kernel BUG at fs/proc/breakme.c:35!
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> ...
> PC is at __bug+0x20/0x2c
>
> With this patch it displays:
>
> kernel BUG at fs/proc/breakme.c:35!
> Internal error: Oops - undefined instruction: 0 [#1] PREEMPT SMP
> ...
> PC is at write_breakme+0xd0/0x1b4
>
> This implementation uses an undefined instruction to implement BUG, and sets up
> a bug table containing the relevant information. Many version of gcc do not
> support %c properly for ARM (inserting a # when they shouldn't) so we work
> around this using distasteful macro magic.
>
> Also the backtrace the message now appears even when CONFIG_ARM_UNWIND is
> defined.
>
> You can fall back to the previous BUG implementation by making GENERIC_BUG
> default to n in arch/arm/Kconfig.
>
> Change-Id: I07d77c832e816f5ad2390e25f466ddf750adecf4
>
> Signed-off-by: Simon Glass <sjg@xxxxxxxxxxxx>
> ---
>  arch/arm/Kconfig              |    4 +++
>  arch/arm/include/asm/bug.h    |   57 +++++++++++++++++++++++++++++++++++++++++
>  arch/arm/kernel/traps.c       |   24 +++++++++++++++++
>  arch/arm/kernel/vmlinux.lds.S |   12 ++++++++
>  4 files changed, 97 insertions(+), 0 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 166efa2..7c8f11c 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -191,6 +191,10 @@ config VECTORS_BASE
>        help
>          The base address of exception vectors.
>
> +config GENERIC_BUG
> +       def_bool y
> +       depends on BUG
> +
>  source "init/Kconfig"
>
>  source "kernel/Kconfig.freezer"
> diff --git a/arch/arm/include/asm/bug.h b/arch/arm/include/asm/bug.h
> index 4d88425..3f8a798 100644
> --- a/arch/arm/include/asm/bug.h
> +++ b/arch/arm/include/asm/bug.h
> @@ -3,6 +3,61 @@
>
>
>  #ifdef CONFIG_BUG
> +
> +#ifdef CONFIG_GENERIC_BUG
> +
> +/*
> + * Use a suitable undefined instruction to use for ARM/Thumb2 bug handling.
> + * We need to be careful not to conflict with those used by other modules and
> + * the register_undef_hook() system.
> + */
> +#ifdef CONFIG_THUMB2_KERNEL
> +#define BUG_INSTR_VALUE 0xde02
> +#define BUG_INSTR_TYPE ".hword "
> +#else
> +#define BUG_INSTR_VALUE 0xe7f001f2
> +#define BUG_INSTR_TYPE ".word "
> +#endif
> +
> +
> +#define BUG() _BUG(__FILE__, __LINE__, BUG_INSTR_VALUE)
> +#define _BUG(file, line, value) __BUG(file, line, value)
> +
> +#ifdef CONFIG_DEBUG_BUGVERBOSE
> +
> +/*
> + * The extra indirection is to ensure that the __FILE__ string comes through
> + * OK. Many version of gcc do not support the asm %c parameter which would be
> + * preferable to this unpleasantness. We use mergeable string sections to
> + * avoid multiple copies of the string appearing in the kernel image.
> + */
> +
> +#define __BUG(__file, __line, __value)                         \
> +do {                                                           \
> +       BUILD_BUG_ON(sizeof(struct bug_entry) != 12);           \
> +       asm volatile("1:\t" BUG_INSTR_TYPE #__value "\n"        \
> +               ".pushsection .rodata.str, \"aMS\", 1\n"        \
> +               "2:\t.asciz " #__file "\n"                      \
> +               ".popsection\n"                                 \
> +               ".pushsection __bug_table,\"a\"\n"              \
> +               "3:\t.word 1b, 2b\n"                            \
> +               "\t.hword " #__line ", 0\n"                     \
> +               ".popsection");                                 \
> +       unreachable();                                          \
> +} while (0)
> +
> +#else  /* not CONFIG_DEBUG_BUGVERBOSE */
> +
> +#define __BUG(__file, __line, __value)                         \
> +do {                                                           \
> +       asm volatile(BUG_INSTR_TYPE #__value);                  \
> +       unreachable();                                          \
> +} while (0)
> +#endif  /* CONFIG_DEBUG_BUGVERBOSE */
> +
> +
> +#else  /* not CONFIG_GENERIC_BUG */
> +
>  #ifdef CONFIG_DEBUG_BUGVERBOSE
>  extern void __bug(const char *file, int line) __attribute__((noreturn));
>
> @@ -16,6 +71,8 @@ extern void __bug(const char *file, int line) __attribute__((noreturn));
>
>  #endif
>
> +#endif  /* CONFIG_GENERIC_BUG */
> +
>  #define HAVE_ARCH_BUG
>  #endif
>
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index ee57640..1199cfe 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -21,6 +21,7 @@
>  #include <linux/kdebug.h>
>  #include <linux/module.h>
>  #include <linux/kexec.h>
> +#include <linux/bug.h>
>  #include <linux/delay.h>
>  #include <linux/init.h>
>
> @@ -163,6 +164,7 @@ static void dump_instr(const char *lvl, struct pt_regs *regs)
>  #ifdef CONFIG_ARM_UNWIND
>  static inline void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
>  {
> +       printk("Backtrace: ");
>        unwind_backtrace(regs, tsk);
>  }
>  #else
> @@ -271,6 +273,8 @@ void die(const char *str, struct pt_regs *regs, int err)
>        spin_lock_irq(&die_lock);
>        console_verbose();
>        bust_spinlocks(1);
> +       if (!user_mode(regs))
> +               report_bug(regs->ARM_pc, regs);
>        ret = __die(str, err, thread, regs);
>
>        if (regs && kexec_should_crash(thread->task))
> @@ -302,6 +306,26 @@ void arm_notify_die(const char *str, struct pt_regs *regs,
>        }
>  }
>
> +#ifdef CONFIG_GENERIC_BUG
> +
> +int is_valid_bugaddr(unsigned long pc)
> +{
> +#ifdef CONFIG_THUMB2_KERNEL
> +       unsigned short bkpt;
> +#else
> +       unsigned long bkpt;
> +#endif
> +
> +       if (pc < PAGE_OFFSET)
> +               return 0;
> +       if (probe_kernel_address((unsigned *)pc, bkpt))
> +               return 0;
> +
> +       return bkpt == BUG_INSTR_VALUE;
> +}
> +
> +#endif
> +
>  static LIST_HEAD(undef_hook);
>  static DEFINE_SPINLOCK(undef_lock);
>
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index 6146279..4f22346 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -80,6 +80,18 @@ SECTIONS
>
>        PERCPU(PAGE_SIZE)
>
> +       /*
> +       * .exit.text is discarded at runtime, not link time, to deal with
> +       *  references from bug_table
> +       */
> +       .exit.text : AT(ADDR(.exit.text)) {
> +               EXIT_TEXT
> +       }
> +
> +       .exit.data : AT(ADDR(.exit.data)) {
> +               EXIT_DATA
> +       }
> +
>  #ifndef CONFIG_XIP_KERNEL
>        . = ALIGN(PAGE_SIZE);
>        __init_end = .;
> --
> 1.7.3.1
>
>
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