[PATCH] rss_stat: Add support to detect RSS updates of external mm

[Joel Fernandes (Google)]

When a process updates the RSS of a different process, the rss_stat
tracepoint appears in the context of the process doing the update. This
can confuse userspace that the RSS of process doing the update is
updated, while in reality a different process's RSS was updated.

This issue happens in reclaim paths such as with direct reclaim or
background reclaim.

This patch adds more information to the tracepoint about whether the mm
being updated belongs to the current process's context (curr field). We
also include a hash of the mm pointer so that the process who the mm
belongs to can be uniquely identified (mm_id field).

Also vsprintf.c is refactored a bit to allow reuse of hashing code.

Reported-by: Ioannis Ilkos <ilkos@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

---
Based on top of the commit in linux-next:
8342d836dc7c ("mm: emit tracepoint when RSS changes")

Google Bug: 140711541

Cc: minchan@xxxxxxxxxx
Cc: primiano@xxxxxxxxxx
Cc: ilkos@xxxxxxxxxx
Cc: fmayer@xxxxxxxxxx
Cc: hjd@xxxxxxxxxx
Cc: joaodias@xxxxxxxxxx
Cc: joelaf@xxxxxxxxxx
Cc: lalitm@xxxxxxxxxx
Cc: rslawik@xxxxxxxxxx
Cc: sspatil@xxxxxxxxxx
Cc: timmurray@xxxxxxxxxx

 include/linux/mm.h          |  8 ++++----
 include/linux/string.h      |  2 ++
 include/trace/events/kmem.h | 32 +++++++++++++++++++++++++++++---
 lib/vsprintf.c              | 36 +++++++++++++++++++++++++-----------
 mm/memory.c                 |  4 ++--
 5 files changed, 62 insertions(+), 20 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 31d8cfb3d988..bfbe65ccffa3 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1643,27 +1643,27 @@ static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
 	return (unsigned long)val;
 }
 
-void mm_trace_rss_stat(int member, long count);
+void mm_trace_rss_stat(struct mm_struct *mm, int member, long count);
 
 static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
 {
 	long count = atomic_long_add_return(value, &mm->rss_stat.count[member]);
 
-	mm_trace_rss_stat(member, count);
+	mm_trace_rss_stat(mm, member, count);
 }
 
 static inline void inc_mm_counter(struct mm_struct *mm, int member)
 {
 	long count = atomic_long_inc_return(&mm->rss_stat.count[member]);
 
-	mm_trace_rss_stat(member, count);
+	mm_trace_rss_stat(mm, member, count);
 }
 
 static inline void dec_mm_counter(struct mm_struct *mm, int member)
 {
 	long count = atomic_long_dec_return(&mm->rss_stat.count[member]);
 
-	mm_trace_rss_stat(member, count);
+	mm_trace_rss_stat(mm, member, count);
 }
 
 /* Optimized variant when page is already known not to be PageAnon */
diff --git a/include/linux/string.h b/include/linux/string.h
index f516cec5277c..6b0e950701d0 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -261,6 +261,8 @@ int bprintf(u32 *bin_buf, size_t size, const char *fmt, ...) __printf(3, 4);
 extern ssize_t memory_read_from_buffer(void *to, size_t count, loff_t *ppos,
 				       const void *from, size_t available);
 
+int ptr_to_hashval(const void *ptr, unsigned long *hashval_out);
+
 /**
  * strstarts - does @str start with @prefix?
  * @str: string to examine
diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index 5a0666bfcf85..ad7e642bd497 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -316,24 +316,50 @@ TRACE_EVENT(mm_page_alloc_extfrag,
 		__entry->change_ownership)
 );
 
+/*
+ * Required for uniquely and securely identifying mm in rss_stat tracepoint.
+ */
+#ifndef __PTR_TO_HASHVAL
+static unsigned int __maybe_unused mm_ptr_to_hash(const void *ptr)
+{
+	int ret;
+	unsigned long hashval;
+
+	ret = ptr_to_hashval(ptr, &hashval);
+	if (ret)
+		return 0;
+
+	/* The hashed value is only 32-bit */
+	return (unsigned int)hashval;
+}
+#define __PTR_TO_HASHVAL
+#endif
+
 TRACE_EVENT(rss_stat,
 
-	TP_PROTO(int member,
+	TP_PROTO(struct mm_struct *mm,
+		int member,
 		long count),
 
-	TP_ARGS(member, count),
+	TP_ARGS(mm, member, count),
 
 	TP_STRUCT__entry(
+		__field(unsigned int, mm_id)
+		__field(unsigned int, curr)
 		__field(int, member)
 		__field(long, size)
 	),
 
 	TP_fast_assign(
+		__entry->mm_id = mm_ptr_to_hash(mm);
+		__entry->curr = !!(current->mm == mm);
 		__entry->member = member;
 		__entry->size = (count << PAGE_SHIFT);
 	),
 
-	TP_printk("member=%d size=%ldB",
+	TP_printk("mm_id=%u curr=%d member=%d size=%ldB",
+		__entry->mm_id,
+		__entry->curr,
 		__entry->member,
 		__entry->size)
 	);
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index dee8fc467fcf..401baaac1813 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -761,11 +761,34 @@ static int __init initialize_ptr_random(void)
 early_initcall(initialize_ptr_random);
 
 /* Maps a pointer to a 32 bit unique identifier. */
+int ptr_to_hashval(const void *ptr, unsigned long *hashval_out)
+{
+	const char *str = sizeof(ptr) == 8 ? "(____ptrval____)" : "(ptrval)";
+	unsigned long hashval;
+
+	if (static_branch_unlikely(&not_filled_random_ptr_key))
+		return -EAGAIN;
+
+#ifdef CONFIG_64BIT
+	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
+	/*
+	 * Mask off the first 32 bits, this makes explicit that we have
+	 * modified the address (and 32 bits is plenty for a unique ID).
+	 */
+	hashval = hashval & 0xffffffff;
+#else
+	hashval = (unsigned long)siphash_1u32((u32)ptr, &ptr_key);
+#endif
+	*hashval_out = hashval;
+	return 0;
+}
+
 static char *ptr_to_id(char *buf, char *end, const void *ptr,
 		       struct printf_spec spec)
 {
 	const char *str = sizeof(ptr) == 8 ? "(____ptrval____)" : "(ptrval)";
 	unsigned long hashval;
+	int ret;
 
 	/* When debugging early boot use non-cryptographically secure hash. */
 	if (unlikely(debug_boot_weak_hash)) {
@@ -773,22 +796,13 @@ static char *ptr_to_id(char *buf, char *end, const void *ptr,
 		return pointer_string(buf, end, (const void *)hashval, spec);
 	}
 
-	if (static_branch_unlikely(&not_filled_random_ptr_key)) {
+	ret = ptr_to_hashval(ptr, &hashval);
+	if (ret) {
 		spec.field_width = 2 * sizeof(ptr);
 		/* string length must be less than default_width */
 		return error_string(buf, end, str, spec);
 	}
 
-#ifdef CONFIG_64BIT
-	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
-	/*
-	 * Mask off the first 32 bits, this makes explicit that we have
-	 * modified the address (and 32 bits is plenty for a unique ID).
-	 */
-	hashval = hashval & 0xffffffff;
-#else
-	hashval = (unsigned long)siphash_1u32((u32)ptr, &ptr_key);
-#endif
 	return pointer_string(buf, end, (const void *)hashval, spec);
 }
 
diff --git a/mm/memory.c b/mm/memory.c
index 7596d625ebd1..d3c9784e6dc1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -154,9 +154,9 @@ static int __init init_zero_pfn(void)
 }
 core_initcall(init_zero_pfn);
 
-void mm_trace_rss_stat(int member, long count)
+void mm_trace_rss_stat(struct mm_struct *mm, int member, long count)
 {
-	trace_rss_stat(member, count);
+	trace_rss_stat(mm, member, count);
 }
 
 #if defined(SPLIT_RSS_COUNTING)
-- 
2.24.0.rc1.363.gb1bccd3e3d-goog