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>

1 files changed, 138 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/test_map_ops.c b/tools/testing/selftests/bpf/progs/test_map_ops.c
new file mode 100644
index 000000000000..b53b46a090c8
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_map_ops.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} hash_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_STACK);
+	__uint(max_entries, 1);
+	__type(value, int);
+} stack_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} array_map SEC(".maps");
+
+const volatile pid_t pid;
+long err = 0;
+
+static u64 callback(u64 map, u64 key, u64 val, u64 ctx, u64 flags)
+{
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getpid")
+int map_update(void *ctx)
+{
+	const int key = 0;
+	const int val = 1;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_update_elem(&hash_map, &key, &val, BPF_NOEXIST);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getppid")
+int map_delete(void *ctx)
+{
+	const int key = 0;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_delete_elem(&hash_map, &key);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getuid")
+int map_push(void *ctx)
+{
+	const int val = 1;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_push_elem(&stack_map, &val, 0);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_geteuid")
+int map_pop(void *ctx)
+{
+	int val;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_pop_elem(&stack_map, &val);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getgid")
+int map_peek(void *ctx)
+{
+	int val;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_peek_elem(&stack_map, &val);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_gettid")
+int map_for_each_pass(void *ctx)
+{
+	const int key = 0;
+	const int val = 1;
+	const u64 flags = 0;
+	int callback_ctx;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	bpf_map_update_elem(&array_map, &key, &val, flags);
+
+	err = bpf_for_each_map_elem(&array_map, callback, &callback_ctx, flags);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getpgid")
+int map_for_each_fail(void *ctx)
+{
+	const int key = 0;
+	const int val = 1;
+	const u64 flags = BPF_NOEXIST;
+	int callback_ctx;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	bpf_map_update_elem(&array_map, &key, &val, flags);
+
+	/* calling for_each with non-zero flags will return error */
+	err = bpf_for_each_map_elem(&array_map, callback, &callback_ctx, flags);
+
+	return 0;
+}