Re: [PATCH 5/5] Powerpc/Watchpoint: Fix length calculation for unaligned target

[Michael Neuling]

On Tue, 2019-06-18 at 09:57 +0530, Ravi Bangoria wrote:
> Watchpoint match range is always doubleword(8 bytes) aligned on
> powerpc. If the given range is crossing doubleword boundary, we
> need to increase the length such that next doubleword also get
> covered. Ex,
> 
>           address   len = 6 bytes
>                 |=========.
>    |------------v--|------v--------|
>    | | | | | | | | | | | | | | | | |
>    |---------------|---------------|
>     <---8 bytes--->
> 
> In such case, current code configures hw as:
>   start_addr = address & ~HW_BREAKPOINT_ALIGN
>   len = 8 bytes
> 
> And thus read/write in last 4 bytes of the given range is ignored.
> Fix this by including next doubleword in the length. Watchpoint
> exception handler already ignores extraneous exceptions, so no
> changes required for that.

Nice catch. Thanks.

I assume this has been broken forever? Should we be CCing stable? If so, it
would be nice to have this self contained (separate from the refactor) so we can
more easily backport it.

Also, can you update 
tools/testing/selftests/powerpc/ptrace/ptrace-hwbreak.c to catch this issue?

A couple more comments below.

> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
> ---
>  arch/powerpc/include/asm/hw_breakpoint.h |  7 ++--
>  arch/powerpc/kernel/hw_breakpoint.c      | 44 +++++++++++++-----------
>  arch/powerpc/kernel/process.c            | 34 ++++++++++++++++--
>  3 files changed, 60 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/hw_breakpoint.h
> b/arch/powerpc/include/asm/hw_breakpoint.h
> index 8acbbdd4a2d5..749a357164d5 100644
> --- a/arch/powerpc/include/asm/hw_breakpoint.h
> +++ b/arch/powerpc/include/asm/hw_breakpoint.h
> @@ -34,6 +34,8 @@ struct arch_hw_breakpoint {
>  #define HW_BRK_TYPE_PRIV_ALL	(HW_BRK_TYPE_USER | HW_BRK_TYPE_KERNEL | \
>  				 HW_BRK_TYPE_HYP)
>  
> +#define HW_BREAKPOINT_ALIGN 0x7
> +
>  #ifdef CONFIG_HAVE_HW_BREAKPOINT
>  #include <linux/kdebug.h>
>  #include <asm/reg.h>
> @@ -45,8 +47,6 @@ struct pmu;
>  struct perf_sample_data;
>  struct task_struct;
>  
> -#define HW_BREAKPOINT_ALIGN 0x7
> -
>  extern int hw_breakpoint_slots(int type);
>  extern int arch_bp_generic_fields(int type, int *gen_bp_type);
>  extern int arch_check_bp_in_kernelspace(struct arch_hw_breakpoint *hw);
> @@ -76,7 +76,8 @@ static inline void hw_breakpoint_disable(void)
>  }
>  extern void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs);
>  int hw_breakpoint_handler(struct die_args *args);
> -
> +extern u16 hw_breakpoint_get_final_len(struct arch_hw_breakpoint *brk,
> +		unsigned long *start_addr, unsigned long *end_addr);
>  extern int set_dawr(struct arch_hw_breakpoint *brk);
>  extern bool dawr_force_enable;
>  static inline bool dawr_enabled(void)
> diff --git a/arch/powerpc/kernel/hw_breakpoint.c
> b/arch/powerpc/kernel/hw_breakpoint.c
> index 36bcf705df65..c122fd55aa44 100644
> --- a/arch/powerpc/kernel/hw_breakpoint.c
> +++ b/arch/powerpc/kernel/hw_breakpoint.c
> @@ -126,6 +126,28 @@ int arch_bp_generic_fields(int type, int *gen_bp_type)
>  	return 0;
>  }
>  
> +/* Maximum len for DABR is 8 bytes and DAWR is 512 bytes */
> +static int hw_breakpoint_validate_len(struct arch_hw_breakpoint *hw)
> +{
> +	u16 length_max = 8;
> +	u16 final_len;
> +	unsigned long start_addr, end_addr;
> +
> +	final_len = hw_breakpoint_get_final_len(hw, &start_addr, &end_addr);
> +
> +	if (dawr_enabled()) {
> +		length_max = 512;
> +		/* DAWR region can't cross 512 bytes boundary */
> +		if ((start_addr >> 9) != (end_addr >> 9))
> +			return -EINVAL;
> +	}
> +
> +	if (final_len > length_max)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
>  /*
>   * Validate the arch-specific HW Breakpoint register settings
>   */
> @@ -133,12 +155,10 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
>  			     const struct perf_event_attr *attr,
>  			     struct arch_hw_breakpoint *hw)
>  {
> -	int length_max;
> -
>  	if (!ppc_breakpoint_available())
>  		return -ENODEV;
>  
> -	if (!bp)
> +	if (!bp || !attr->bp_len)
>  		return -EINVAL;
>  
>  	hw->type = HW_BRK_TYPE_TRANSLATE;
> @@ -160,23 +180,7 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
>  	hw->address = attr->bp_addr;
>  	hw->len = attr->bp_len;
>  
> -	length_max = 8; /* DABR */
> -	if (dawr_enabled()) {
> -		length_max = 512 ; /* 64 doublewords */
> -		/* DAWR region can't cross 512 bytes boundary */
> -		if ((hw->address >> 9) != ((hw->address + hw->len - 1) >> 9))
> -			return -EINVAL;
> -	}
> -
> -	/*
> -	 * Since breakpoint length can be a maximum of length_max and
> -	 * breakpoint addresses are aligned to nearest double-word
> -	 * HW_BREAKPOINT_ALIGN by rounding off to the lower address,
> -	 * the 'symbolsize' should satisfy the check below.
> -	 */
> -	if (hw->len > (length_max - (hw->address & HW_BREAKPOINT_ALIGN)))
> -		return -EINVAL;
> -	return 0;
> +	return hw_breakpoint_validate_len(hw);
>  }
>  
>  /*
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 265fac9fb3a4..159aaa70de46 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -802,9 +802,39 @@ static int disable_dawr(void)
>  	return 0;
>  }
>  
> +/*
> + * Watchpoint match range is always doubleword(8 bytes) aligned on
> + * powerpc. If the given range is crossing doubleword boundary, we
> + * need to increase the length such that next doubleword also get
> + * covered. Ex,
> + *
> + *          address   len = 6 bytes
> + *                |=========.
> + *   |------------v--|------v--------|
> + *   | | | | | | | | | | | | | | | | |
> + *   |---------------|---------------|
> + *    <---8 bytes--->
> + *
> + * In this case, we should configure hw as:
> + *   start_addr = address & ~HW_BREAKPOINT_ALIGN
> + *   len = 16 bytes
> + *
> + * @start_addr and @end_addr are inclusive.
> + */
> +u16 hw_breakpoint_get_final_len(struct arch_hw_breakpoint *brk,
> +				unsigned long *start_addr,
> +				unsigned long *end_addr)

I don't really like this.  "final" is not a good name. Something like hardware
would be better.

Also, can you put the start_addr and end addr in the arch_hw_breakpoint rather
than doing what you have above.  Call them hw_start_addr, hw_end_addr.

We could even set these two new addresses where we set the set of
arch_hw_breakpoint rather than having this late call.

> +{
> +	*start_addr = brk->address & ~HW_BREAKPOINT_ALIGN;
> +	*end_addr = (brk->address + brk->len - 1) | HW_BREAKPOINT_ALIGN;
> +	return *end_addr - *start_addr + 1;
> +}
> +
>  int set_dawr(struct arch_hw_breakpoint *brk)
>  {
>  	unsigned long dawr, dawrx, mrd;
> +	unsigned long start_addr, end_addr;
> +	u16 final_len;
>  
>  	if (brk->type == HW_BRK_TYPE_DISABLE)
>  		return disable_dawr();
> @@ -815,8 +845,8 @@ int set_dawr(struct arch_hw_breakpoint *brk)
>  	dawrx |= ((brk->type & HW_BRK_TYPE_TRANSLATE) >> 2) << (63 - 59);
>  	dawrx |= (brk->type & HW_BRK_TYPE_PRIV_ALL) >> 3;
>  
> -	/* brk->len is in bytes. */
> -	mrd = ((brk->len + 7) >> 3) - 1;
> +	final_len = hw_breakpoint_get_final_len(brk, &start_addr, &end_addr);

Again, hardware length, or something other than "final"

> +	mrd = ((final_len + 7) >> 3) - 1;
>  	dawrx |= (mrd & 0x3f) << (63 - 53);
>  
>  	if (ppc_md.set_dawr)