Re: [PATCH v11 3/8] powerpc/perf: consolidate read_user_stack_32

[Nicholas Piggin <npiggin@gmail.com>]

Michal Suchánek's on March 25, 2020 5:38 am:
> On Tue, Mar 24, 2020 at 06:48:20PM +1000, Nicholas Piggin wrote:
>> Michal Suchanek's on March 19, 2020 10:19 pm:
>> > There are two almost identical copies for 32bit and 64bit.
>> > 
>> > The function is used only in 32bit code which will be split out in next
>> > patch so consolidate to one function.
>> > 
>> > Signed-off-by: Michal Suchanek <msuchanek@suse.de>
>> > Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
>> > ---
>> > v6:  new patch
>> > v8:  move the consolidated function out of the ifdef block.
>> > v11: rebase on top of def0bfdbd603
>> > ---
>> >  arch/powerpc/perf/callchain.c | 48 +++++++++++++++++------------------
>> >  1 file changed, 24 insertions(+), 24 deletions(-)
>> > 
>> > diff --git a/arch/powerpc/perf/callchain.c b/arch/powerpc/perf/callchain.c
>> > index cbc251981209..c9a78c6e4361 100644
>> > --- a/arch/powerpc/perf/callchain.c
>> > +++ b/arch/powerpc/perf/callchain.c
>> > @@ -161,18 +161,6 @@ static int read_user_stack_64(unsigned long __user *ptr, unsigned long *ret)
>> >  	return read_user_stack_slow(ptr, ret, 8);
>> >  }
>> >  
>> > -static int read_user_stack_32(unsigned int __user *ptr, unsigned int *ret)
>> > -{
>> > -	if ((unsigned long)ptr > TASK_SIZE - sizeof(unsigned int) ||
>> > -	    ((unsigned long)ptr & 3))
>> > -		return -EFAULT;
>> > -
>> > -	if (!probe_user_read(ret, ptr, sizeof(*ret)))
>> > -		return 0;
>> > -
>> > -	return read_user_stack_slow(ptr, ret, 4);
>> > -}
>> > -
>> >  static inline int valid_user_sp(unsigned long sp, int is_64)
>> >  {
>> >  	if (!sp || (sp & 7) || sp > (is_64 ? TASK_SIZE : 0x100000000UL) - 32)
>> > @@ -277,19 +265,9 @@ static void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
>> >  }
>> >  
>> >  #else  /* CONFIG_PPC64 */
>> > -/*
>> > - * On 32-bit we just access the address and let hash_page create a
>> > - * HPTE if necessary, so there is no need to fall back to reading
>> > - * the page tables.  Since this is called at interrupt level,
>> > - * do_page_fault() won't treat a DSI as a page fault.
>> > - */
>> > -static int read_user_stack_32(unsigned int __user *ptr, unsigned int *ret)
>> > +static int read_user_stack_slow(void __user *ptr, void *buf, int nb)
>> >  {
>> > -	if ((unsigned long)ptr > TASK_SIZE - sizeof(unsigned int) ||
>> > -	    ((unsigned long)ptr & 3))
>> > -		return -EFAULT;
>> > -
>> > -	return probe_user_read(ret, ptr, sizeof(*ret));
>> > +	return 0;
>> >  }
>> >  
>> >  static inline void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
>> > @@ -312,6 +290,28 @@ static inline int valid_user_sp(unsigned long sp, int is_64)
>> >  
>> >  #endif /* CONFIG_PPC64 */
>> >  
>> > +/*
>> > + * On 32-bit we just access the address and let hash_page create a
>> > + * HPTE if necessary, so there is no need to fall back to reading
>> > + * the page tables.  Since this is called at interrupt level,
>> > + * do_page_fault() won't treat a DSI as a page fault.
>> > + */
>> 
>> The comment is actually probably better to stay in the 32-bit
>> read_user_stack_slow implementation. Is that function defined
>> on 32-bit purely so that you can use IS_ENABLED()? In that case
> It documents the IS_ENABLED() and that's where it is. The 32bit
> definition is only a technical detail.

Sorry for the late reply, busy trying to fix bugs in the C rewrite
series. I don't think it is the right place, it should be in the
ppc32 implementation detail. ppc64 has an equivalent comment at the
top of its read_user_stack functions.

>> I would prefer to put a BUG() there which makes it self documenting.
> Which will cause checkpatch complaints about introducing new BUG() which
> is frowned on.

It's fine in this case, that warning is about not introducing
runtime bugs, but this wouldn't be.

But... I actually don't like adding read_user_stack_slow on 32-bit
and especially not just to make IS_ENABLED work.

IMO this would be better if you really want to consolidate it

---

diff --git a/arch/powerpc/perf/callchain.c b/arch/powerpc/perf/callchain.c
index cbc251981209..ca3a599b3f54 100644
--- a/arch/powerpc/perf/callchain.c
+++ b/arch/powerpc/perf/callchain.c
@@ -108,7 +108,7 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
  * interrupt context, so if the access faults, we read the page tables
  * to find which page (if any) is mapped and access it directly.
  */
-static int read_user_stack_slow(void __user *ptr, void *buf, int nb)
+static int read_user_stack_slow(const void __user *ptr, void *buf, int nb)
 {
 	int ret = -EFAULT;
 	pgd_t *pgdir;
@@ -149,28 +149,21 @@ static int read_user_stack_slow(void __user *ptr, void *buf, int nb)
 	return ret;
 }
 
-static int read_user_stack_64(unsigned long __user *ptr, unsigned long *ret)
+static int __read_user_stack(const void __user *ptr, void *ret, size_t size)
 {
-	if ((unsigned long)ptr > TASK_SIZE - sizeof(unsigned long) ||
-	    ((unsigned long)ptr & 7))
+	if ((unsigned long)ptr > TASK_SIZE - size ||
+	    ((unsigned long)ptr & (size - 1)))
 		return -EFAULT;
 
-	if (!probe_user_read(ret, ptr, sizeof(*ret)))
+	if (!probe_user_read(ret, ptr, size))
 		return 0;
 
-	return read_user_stack_slow(ptr, ret, 8);
+	return read_user_stack_slow(ptr, ret, size);
 }
 
-static int read_user_stack_32(unsigned int __user *ptr, unsigned int *ret)
+static int read_user_stack_64(unsigned long __user *ptr, unsigned long *ret)
 {
-	if ((unsigned long)ptr > TASK_SIZE - sizeof(unsigned int) ||
-	    ((unsigned long)ptr & 3))
-		return -EFAULT;
-
-	if (!probe_user_read(ret, ptr, sizeof(*ret)))
-		return 0;
-
-	return read_user_stack_slow(ptr, ret, 4);
+	return __read_user_stack(ptr, ret, sizeof(*ret));
 }
 
 static inline int valid_user_sp(unsigned long sp, int is_64)
@@ -283,13 +276,13 @@ static void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
  * the page tables.  Since this is called at interrupt level,
  * do_page_fault() won't treat a DSI as a page fault.
  */
-static int read_user_stack_32(unsigned int __user *ptr, unsigned int *ret)
+static int __read_user_stack(const void __user *ptr, void *ret, size_t size)
 {
-	if ((unsigned long)ptr > TASK_SIZE - sizeof(unsigned int) ||
-	    ((unsigned long)ptr & 3))
+	if ((unsigned long)ptr > TASK_SIZE - size ||
+	    ((unsigned long)ptr & (size - 1)))
 		return -EFAULT;
 
-	return probe_user_read(ret, ptr, sizeof(*ret));
+	return probe_user_read(ret, ptr, size);
 }
 
 static inline void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
@@ -312,6 +305,11 @@ static inline int valid_user_sp(unsigned long sp, int is_64)
 
 #endif /* CONFIG_PPC64 */
 
+static int read_user_stack_32(unsigned int __user *ptr, unsigned int *ret)
+{
+	return __read_user_stack(ptr, ret, sizeof(*ret));
+}
+
 /*
  * Layout for non-RT signal frames
  */