[RFC PATCH v1 13/57] bpf: Remove PAGE_SIZE compile-time constant assumption

Ryan Roberts
2024-10-14T11:42:08Z

To prepare for supporting boot-time page size selection, refactor code
to remove assumptions about PAGE_SIZE being compile-time constant. Code
intended to be equivalent when compile-time page size is active.

Refactor "struct bpf_ringbuf" so that consumer_pos, producer_pos,
pending_pos and data are no longer embedded at (static) page offsets
within the struct. This can't work for boot-time page size because the
page size isn't known at compile-time. Instead, only define the meta
data in the struct, along with pointers to those values. At "struct
bpf_ringbuf" allocation time, the extra pages are allocated at the end
and the pointers are initialized to point to the correct locations.

Additionally, only expose the __PAGE_SIZE enum to BTF for compile-time
page size builds. We don't know the page size at compile-time for
boot-time builds. NOTE: This may need some extra thought; perhaps
__PAGE_SIZE should be exposed as 0 in this case? And/or perhaps
__PAGE_SIZE_MIN/__PAGE_SIZE_MAX should be exposed? And there would need
to be a runtime mechanism for querying the page size (e.g.
getpagesize()).

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

kernel/bpf/core.c | 9 ++++++--
kernel/bpf/ringbuf.c | 54 ++++++++++++++++++++++++--------------------
2 files changed, 37 insertions(+), 26 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 7ee62e38faf0e..485875aa78e63 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -89,10 +89,15 @@ void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb, int k, uns
return NULL;
}

-/* tell bpf programs that include vmlinux.h kernel's PAGE_SIZE */
+/*
+ * tell bpf programs that include vmlinux.h kernel's PAGE_SIZE. We can only do
+ * this for compile-time PAGE_SIZE builds.
+ */
+#if PAGE_SIZE_MIN == PAGE_SIZE_MAX
enum page_size_enum {
__PAGE_SIZE = PAGE_SIZE
};
+#endif

struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flags)
{
@@ -100,7 +105,7 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
struct bpf_prog_aux *aux;
struct bpf_prog *fp;

- size = round_up(size, __PAGE_SIZE);
+ size = round_up(size, PAGE_SIZE);
fp = __vmalloc(size, gfp_flags);
if (fp == NULL)
return NULL;
diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
index e20b90c361316..8e4093ddbc638 100644
--- a/kernel/bpf/ringbuf.c
+++ b/kernel/bpf/ringbuf.c
@@ -14,9 +14,9 @@

#define RINGBUF_CREATE_FLAG_MASK (BPF_F_NUMA_NODE)

-/* non-mmap()'able part of bpf_ringbuf (everything up to consumer page) */
+/* non-mmap()'able part of bpf_ringbuf (everything defined in struct) */
#define RINGBUF_PGOFF \
- (offsetof(struct bpf_ringbuf, consumer_pos) >> PAGE_SHIFT)
+ (PAGE_ALIGN(sizeof(struct bpf_ringbuf)) >> PAGE_SHIFT)
/* consumer page and producer page */
#define RINGBUF_POS_PAGES 2
#define RINGBUF_NR_META_PAGES (RINGBUF_PGOFF + RINGBUF_POS_PAGES)
@@ -69,10 +69,10 @@ struct bpf_ringbuf {
* validate each sample to ensure that they're correctly formatted, and
* fully contained within the ring buffer.
*/
- unsigned long consumer_pos __aligned(PAGE_SIZE);
- unsigned long producer_pos __aligned(PAGE_SIZE);
- unsigned long pending_pos;
- char data[] __aligned(PAGE_SIZE);
+ unsigned long *consumer_pos;
+ unsigned long *producer_pos;
+ unsigned long *pending_pos;
+ char *data;
};

struct bpf_ringbuf_map {
@@ -134,9 +134,15 @@ static struct bpf_ringbuf *bpf_ringbuf_area_alloc(size_t data_sz, int numa_node)
rb = vmap(pages, nr_meta_pages + 2 * nr_data_pages,
VM_MAP | VM_USERMAP, PAGE_KERNEL);
if (rb) {
+ void *base = rb;
+
kmemleak_not_leak(pages);
rb->pages = pages;
rb->nr_pages = nr_pages;
+ rb->consumer_pos = (unsigned long *)(base + PAGE_SIZE * RINGBUF_PGOFF);
+ rb->producer_pos = (unsigned long *)(base + PAGE_SIZE * (RINGBUF_PGOFF + 1));
+ rb->pending_pos = rb->producer_pos + 1;
+ rb->data = base + PAGE_SIZE * nr_meta_pages;
return rb;
}

@@ -179,9 +185,9 @@ static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node)
init_irq_work(&rb->work, bpf_ringbuf_notify);

rb->mask = data_sz - 1;
- rb->consumer_pos = 0;
- rb->producer_pos = 0;
- rb->pending_pos = 0;
+ *rb->consumer_pos = 0;
+ *rb->producer_pos = 0;
+ *rb->pending_pos = 0;

return rb;
}
@@ -300,8 +306,8 @@ static unsigned long ringbuf_avail_data_sz(struct bpf_ringbuf *rb)
{
unsigned long cons_pos, prod_pos;

- cons_pos = smp_load_acquire(&rb->consumer_pos);
- prod_pos = smp_load_acquire(&rb->producer_pos);
+ cons_pos = smp_load_acquire(rb->consumer_pos);
+ prod_pos = smp_load_acquire(rb->producer_pos);
return prod_pos - cons_pos;
}

@@ -418,7 +424,7 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
if (len > ringbuf_total_data_sz(rb))
return NULL;

- cons_pos = smp_load_acquire(&rb->consumer_pos);
+ cons_pos = smp_load_acquire(rb->consumer_pos);

if (in_nmi()) {
if (!spin_trylock_irqsave(&rb->spinlock, flags))
@@ -427,8 +433,8 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
spin_lock_irqsave(&rb->spinlock, flags);
}

- pend_pos = rb->pending_pos;
- prod_pos = rb->producer_pos;
+ pend_pos = *rb->pending_pos;
+ prod_pos = *rb->producer_pos;
new_prod_pos = prod_pos + len;

while (pend_pos < prod_pos) {
@@ -440,7 +446,7 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
tmp_size = round_up(tmp_size + BPF_RINGBUF_HDR_SZ, 8);
pend_pos += tmp_size;
}
- rb->pending_pos = pend_pos;
+ *rb->pending_pos = pend_pos;

/* check for out of ringbuf space:
* - by ensuring producer position doesn't advance more than
@@ -460,7 +466,7 @@ static void *__bpf_ringbuf_reserve(struct bpf_ringbuf *rb, u64 size)
hdr->pg_off = pg_off;

/* pairs with consumer's smp_load_acquire() */
- smp_store_release(&rb->producer_pos, new_prod_pos);
+ smp_store_release(rb->producer_pos, new_prod_pos);

spin_unlock_irqrestore(&rb->spinlock, flags);

@@ -506,7 +512,7 @@ static void bpf_ringbuf_commit(void *sample, u64 flags, bool discard)
* new data availability
*/
rec_pos = (void *)hdr - (void *)rb->data;
- cons_pos = smp_load_acquire(&rb->consumer_pos) & rb->mask;
+ cons_pos = smp_load_acquire(rb->consumer_pos) & rb->mask;

if (flags & BPF_RB_FORCE_WAKEUP)
irq_work_queue(&rb->work);
@@ -580,9 +586,9 @@ BPF_CALL_2(bpf_ringbuf_query, struct bpf_map *, map, u64, flags)
case BPF_RB_RING_SIZE:
return ringbuf_total_data_sz(rb);
case BPF_RB_CONS_POS:
- return smp_load_acquire(&rb->consumer_pos);
+ return smp_load_acquire(rb->consumer_pos);
case BPF_RB_PROD_POS:
- return smp_load_acquire(&rb->producer_pos);
+ return smp_load_acquire(rb->producer_pos);
default:
return 0;
}
@@ -680,12 +686,12 @@ static int __bpf_user_ringbuf_peek(struct bpf_ringbuf *rb, void **sample, u32 *s
u64 cons_pos, prod_pos;

/* Synchronizes with smp_store_release() in user-space producer. */
- prod_pos = smp_load_acquire(&rb->producer_pos);
+ prod_pos = smp_load_acquire(rb->producer_pos);
if (prod_pos % 8)
return -EINVAL;

/* Synchronizes with smp_store_release() in __bpf_user_ringbuf_sample_release() */
- cons_pos = smp_load_acquire(&rb->consumer_pos);
+ cons_pos = smp_load_acquire(rb->consumer_pos);
if (cons_pos >= prod_pos)
return -ENODATA;

@@ -715,7 +721,7 @@ static int __bpf_user_ringbuf_peek(struct bpf_ringbuf *rb, void **sample, u32 *s
* Update the consumer pos, and return -EAGAIN so the caller
* knows to skip this sample and try to read the next one.
*/
- smp_store_release(&rb->consumer_pos, cons_pos + total_len);
+ smp_store_release(rb->consumer_pos, cons_pos + total_len);
return -EAGAIN;
}

@@ -737,9 +743,9 @@ static void __bpf_user_ringbuf_sample_release(struct bpf_ringbuf *rb, size_t siz
* prevents another task from writing to consumer_pos after it was read
* by this task with smp_load_acquire() in __bpf_user_ringbuf_peek().
*/
- consumer_pos = rb->consumer_pos;
+ consumer_pos = *rb->consumer_pos;
/* Synchronizes with smp_load_acquire() in user-space producer. */
- smp_store_release(&rb->consumer_pos, consumer_pos + rounded_size);
+ smp_store_release(rb->consumer_pos, consumer_pos + rounded_size);
}

BPF_CALL_4(bpf_user_ringbuf_drain, struct bpf_map *, map,
--
2.43.0