Merge branch 'error checking where helpers call bpf_map_ops'

JP Kobryn says:

====================

Within bpf programs, the bpf helper functions can make inline calls to
kernel functions. In this scenario there can be a disconnect between the
register the kernel function writes a return value to and the register the
bpf program uses to evaluate that return value.

As an example, this bpf code:

long err = bpf_map_update_elem(...);
if (err && err != -EEXIST)
	// got some error other than -EEXIST

...can result in the bpf assembly:

; err = bpf_map_update_elem(&mymap, &key, &val, BPF_NOEXIST);
  37:	movabs $0xffff976a10730400,%rdi
  41:	mov    $0x1,%ecx
  46:	call   0xffffffffe103291c	; htab_map_update_elem
; if (err && err != -EEXIST) {
  4b:	cmp    $0xffffffffffffffef,%rax ; cmp -EEXIST,%rax
  4f:	je     0x000000000000008e
  51:	test   %rax,%rax
  54:	je     0x000000000000008e

The compare operation here evaluates %rax, while in the preceding call to
htab_map_update_elem the corresponding assembly returns -EEXIST via %eax
(the lower 32 bits of %rax):

movl $0xffffffef, %r9d
...
movl %r9d, %eax

...since it's returning int (32-bit). So the resulting comparison becomes:

cmp $0xffffffffffffffef, $0x00000000ffffffef

...making it not possible to check for negative errors or specific errors,
since the sign value is left at the 32nd bit. It means in the original
example, the conditional branch will be entered even when the error is
-EEXIST, which was not intended.

The selftests added cover these cases for the different bpf_map_ops
functions. When the second patch is applied, changing the return type of
those functions to long, the comparison works as intended and the tests
pass.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 10 insertions, 10 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 3ef98fb92987..ec0df059f562 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -96,11 +96,11 @@ struct bpf_map_ops {
 
 	/* funcs callable from userspace and from eBPF programs */
 	void *(*map_lookup_elem)(struct bpf_map *map, void *key);
-	int (*map_update_elem)(struct bpf_map *map, void *key, void *value, u64 flags);
-	int (*map_delete_elem)(struct bpf_map *map, void *key);
-	int (*map_push_elem)(struct bpf_map *map, void *value, u64 flags);
-	int (*map_pop_elem)(struct bpf_map *map, void *value);
-	int (*map_peek_elem)(struct bpf_map *map, void *value);
+	long (*map_update_elem)(struct bpf_map *map, void *key, void *value, u64 flags);
+	long (*map_delete_elem)(struct bpf_map *map, void *key);
+	long (*map_push_elem)(struct bpf_map *map, void *value, u64 flags);
+	long (*map_pop_elem)(struct bpf_map *map, void *value);
+	long (*map_peek_elem)(struct bpf_map *map, void *value);
 	void *(*map_lookup_percpu_elem)(struct bpf_map *map, void *key, u32 cpu);
 
 	/* funcs called by prog_array and perf_event_array map */
@@ -139,7 +139,7 @@ struct bpf_map_ops {
 	struct bpf_local_storage __rcu ** (*map_owner_storage_ptr)(void *owner);
 
 	/* Misc helpers.*/
-	int (*map_redirect)(struct bpf_map *map, u64 key, u64 flags);
+	long (*map_redirect)(struct bpf_map *map, u64 key, u64 flags);
 
 	/* map_meta_equal must be implemented for maps that can be
 	 * used as an inner map.  It is a runtime check to ensure
@@ -157,7 +157,7 @@ struct bpf_map_ops {
 	int (*map_set_for_each_callback_args)(struct bpf_verifier_env *env,
 					      struct bpf_func_state *caller,
 					      struct bpf_func_state *callee);
-	int (*map_for_each_callback)(struct bpf_map *map,
+	long (*map_for_each_callback)(struct bpf_map *map,
 				     bpf_callback_t callback_fn,
 				     void *callback_ctx, u64 flags);
 
diff --git a/include/linux/filter.h b/include/linux/filter.h
index efa5d4a1677e..23c08c31bea9 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1504,9 +1504,9 @@ static inline bool bpf_sk_lookup_run_v6(struct net *net, int protocol,
 }
 #endif /* IS_ENABLED(CONFIG_IPV6) */
 
-static __always_inline int __bpf_xdp_redirect_map(struct bpf_map *map, u64 index,
-						  u64 flags, const u64 flag_mask,
-						  void *lookup_elem(struct bpf_map *map, u32 key))
+static __always_inline long __bpf_xdp_redirect_map(struct bpf_map *map, u64 index,
+						   u64 flags, const u64 flag_mask,
+						   void *lookup_elem(struct bpf_map *map, u32 key))
 {
 	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
 	const u64 action_mask = XDP_ABORTED | XDP_DROP | XDP_PASS | XDP_TX;