[PATCH v6 02/44] stackdepot: reserve 5 extra bits in depot_stack_handle_t

[Alexander Potapenko <glider@google.com>]

Some users (currently only KMSAN) may want to use spare bits in
depot_stack_handle_t. Let them do so by adding @extra_bits to
__stack_depot_save() to store arbitrary flags, and providing
stack_depot_get_extra_bits() to retrieve those flags.

Also adapt KASAN to the new prototype by passing extra_bits=0, as KASAN
does not intend to store additional information in the stack handle.

Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Marco Elver <elver@google.com>

---
v4:
 -- per Marco Elver's request, fold "kasan: common: adapt to the new
    prototype of __stack_depot_save()" into this patch to prevent
    bisection breakages.

Link: https://linux-review.googlesource.com/id/I0587f6c777667864768daf07821d594bce6d8ff9
---
 include/linux/stackdepot.h |  8 ++++++++
 lib/stackdepot.c           | 29 ++++++++++++++++++++++++-----
 mm/kasan/common.c          |  2 +-
 3 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
index bc2797955de90..9ca7798d7a318 100644
--- a/include/linux/stackdepot.h
+++ b/include/linux/stackdepot.h
@@ -14,9 +14,15 @@
 #include <linux/gfp.h>
 
 typedef u32 depot_stack_handle_t;
+/*
+ * Number of bits in the handle that stack depot doesn't use. Users may store
+ * information in them.
+ */
+#define STACK_DEPOT_EXTRA_BITS 5
 
 depot_stack_handle_t __stack_depot_save(unsigned long *entries,
 					unsigned int nr_entries,
+					unsigned int extra_bits,
 					gfp_t gfp_flags, bool can_alloc);
 
 /*
@@ -59,6 +65,8 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
 unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 			       unsigned long **entries);
 
+unsigned int stack_depot_get_extra_bits(depot_stack_handle_t handle);
+
 int stack_depot_snprint(depot_stack_handle_t handle, char *buf, size_t size,
 		       int spaces);
 
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index e73fda23388d8..79e894cf84064 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -43,7 +43,8 @@
 #define STACK_ALLOC_OFFSET_BITS (STACK_ALLOC_ORDER + PAGE_SHIFT - \
 					STACK_ALLOC_ALIGN)
 #define STACK_ALLOC_INDEX_BITS (DEPOT_STACK_BITS - \
-		STACK_ALLOC_NULL_PROTECTION_BITS - STACK_ALLOC_OFFSET_BITS)
+		STACK_ALLOC_NULL_PROTECTION_BITS - \
+		STACK_ALLOC_OFFSET_BITS - STACK_DEPOT_EXTRA_BITS)
 #define STACK_ALLOC_SLABS_CAP 8192
 #define STACK_ALLOC_MAX_SLABS \
 	(((1LL << (STACK_ALLOC_INDEX_BITS)) < STACK_ALLOC_SLABS_CAP) ? \
@@ -56,6 +57,7 @@ union handle_parts {
 		u32 slabindex : STACK_ALLOC_INDEX_BITS;
 		u32 offset : STACK_ALLOC_OFFSET_BITS;
 		u32 valid : STACK_ALLOC_NULL_PROTECTION_BITS;
+		u32 extra : STACK_DEPOT_EXTRA_BITS;
 	};
 };
 
@@ -77,6 +79,14 @@ static int next_slab_inited;
 static size_t depot_offset;
 static DEFINE_RAW_SPINLOCK(depot_lock);
 
+unsigned int stack_depot_get_extra_bits(depot_stack_handle_t handle)
+{
+	union handle_parts parts = { .handle = handle };
+
+	return parts.extra;
+}
+EXPORT_SYMBOL(stack_depot_get_extra_bits);
+
 static bool init_stack_slab(void **prealloc)
 {
 	if (!*prealloc)
@@ -140,6 +150,7 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 	stack->handle.slabindex = depot_index;
 	stack->handle.offset = depot_offset >> STACK_ALLOC_ALIGN;
 	stack->handle.valid = 1;
+	stack->handle.extra = 0;
 	memcpy(stack->entries, entries, flex_array_size(stack, entries, size));
 	depot_offset += required_size;
 
@@ -382,6 +393,7 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
  *
  * @entries:		Pointer to storage array
  * @nr_entries:		Size of the storage array
+ * @extra_bits:		Flags to store in unused bits of depot_stack_handle_t
  * @alloc_flags:	Allocation gfp flags
  * @can_alloc:		Allocate stack slabs (increased chance of failure if false)
  *
@@ -393,6 +405,10 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
  * If the stack trace in @entries is from an interrupt, only the portion up to
  * interrupt entry is saved.
  *
+ * Additional opaque flags can be passed in @extra_bits, stored in the unused
+ * bits of the stack handle, and retrieved using stack_depot_get_extra_bits()
+ * without calling stack_depot_fetch().
+ *
  * Context: Any context, but setting @can_alloc to %false is required if
  *          alloc_pages() cannot be used from the current context. Currently
  *          this is the case from contexts where neither %GFP_ATOMIC nor
@@ -402,10 +418,11 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
  */
 depot_stack_handle_t __stack_depot_save(unsigned long *entries,
 					unsigned int nr_entries,
+					unsigned int extra_bits,
 					gfp_t alloc_flags, bool can_alloc)
 {
 	struct stack_record *found = NULL, **bucket;
-	depot_stack_handle_t retval = 0;
+	union handle_parts retval = { .handle = 0 };
 	struct page *page = NULL;
 	void *prealloc = NULL;
 	unsigned long flags;
@@ -489,9 +506,11 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
 		free_pages((unsigned long)prealloc, STACK_ALLOC_ORDER);
 	}
 	if (found)
-		retval = found->handle.handle;
+		retval.handle = found->handle.handle;
 fast_exit:
-	return retval;
+	retval.extra = extra_bits;
+
+	return retval.handle;
 }
 EXPORT_SYMBOL_GPL(__stack_depot_save);
 
@@ -511,6 +530,6 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
 				      unsigned int nr_entries,
 				      gfp_t alloc_flags)
 {
-	return __stack_depot_save(entries, nr_entries, alloc_flags, true);
+	return __stack_depot_save(entries, nr_entries, 0, alloc_flags, true);
 }
 EXPORT_SYMBOL_GPL(stack_depot_save);
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 69f583855c8be..94caa2d46a327 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -36,7 +36,7 @@ depot_stack_handle_t kasan_save_stack(gfp_t flags, bool can_alloc)
 	unsigned int nr_entries;
 
 	nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
-	return __stack_depot_save(entries, nr_entries, flags, can_alloc);
+	return __stack_depot_save(entries, nr_entries, 0, flags, can_alloc);
 }
 
 void kasan_set_track(struct kasan_track *track, gfp_t flags)
-- 
2.37.2.789.g6183377224-goog