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    SubjectRe: [PATCH v4 02/45] stackdepot: reserve 5 extra bits in depot_stack_handle_t
    On Fri, 1 Jul 2022 at 16:23, Alexander Potapenko <glider@google.com> wrote:
    >
    > 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 5ca0d086ef4a3..3d1dbdd5a87f6 100644
    > --- a/lib/stackdepot.c
    > +++ b/lib/stackdepot.c
    > @@ -42,7 +42,8 @@
    > #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) ? \
    > @@ -55,6 +56,7 @@ 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;
    > };
    > };
    >
    > @@ -76,6 +78,14 @@ 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)
    > @@ -139,6 +149,7 @@ 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;
    >
    > @@ -343,6 +354,7 @@ 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)
    > *
    > @@ -354,6 +366,10 @@ 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
    > @@ -363,10 +379,11 @@ 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;
    > @@ -450,9 +467,11 @@ 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);
    >
    > @@ -472,6 +491,6 @@ 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 c40c0e7b3b5f1..ba4fceeec173c 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)
    > --
    > 2.37.0.rc0.161.g10f37bed90-goog
    >

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