bpf: Introduce BPF trampoline

Introduce BPF trampoline concept to allow kernel code to call into BPF programs
with practically zero overhead.  The trampoline generation logic is
architecture dependent.  It's converting native calling convention into BPF
calling convention.  BPF ISA is 64-bit (even on 32-bit architectures). The
registers R1 to R5 are used to pass arguments into BPF functions. The main BPF
program accepts only single argument "ctx" in R1. Whereas CPU native calling
convention is different. x86-64 is passing first 6 arguments in registers
and the rest on the stack. x86-32 is passing first 3 arguments in registers.
sparc64 is passing first 6 in registers. And so on.

The trampolines between BPF and kernel already exist.  BPF_CALL_x macros in
include/linux/filter.h statically compile trampolines from BPF into kernel
helpers. They convert up to five u64 arguments into kernel C pointers and
integers. On 64-bit architectures this BPF_to_kernel trampolines are nops. On
32-bit architecture they're meaningful.

The opposite job kernel_to_BPF trampolines is done by CAST_TO_U64 macros and
__bpf_trace_##call() shim functions in include/trace/bpf_probe.h. They convert
kernel function arguments into array of u64s that BPF program consumes via
R1=ctx pointer.

This patch set is doing the same job as __bpf_trace_##call() static
trampolines, but dynamically for any kernel function. There are ~22k global
kernel functions that are attachable via nop at function entry. The function
arguments and types are described in BTF.  The job of btf_distill_func_proto()
function is to extract useful information from BTF into "function model" that
architecture dependent trampoline generators will use to generate assembly code
to cast kernel function arguments into array of u64s.  For example the kernel
function eth_type_trans has two pointers. They will be casted to u64 and stored
into stack of generated trampoline. The pointer to that stack space will be
passed into BPF program in R1. On x86-64 such generated trampoline will consume
16 bytes of stack and two stores of %rdi and %rsi into stack. The verifier will
make sure that only two u64 are accessed read-only by BPF program. The verifier
will also recognize the precise type of the pointers being accessed and will
not allow typecasting of the pointer to a different type within BPF program.

The tracing use case in the datacenter demonstrated that certain key kernel
functions have (like tcp_retransmit_skb) have 2 or more kprobes that are always
active.  Other functions have both kprobe and kretprobe.  So it is essential to
keep both kernel code and BPF programs executing at maximum speed. Hence
generated BPF trampoline is re-generated every time new program is attached or
detached to maintain maximum performance.

To avoid the high cost of retpoline the attached BPF programs are called
directly. __bpf_prog_enter/exit() are used to support per-program execution
stats.  In the future this logic will be optimized further by adding support
for bpf_stats_enabled_key inside generated assembly code. Introduction of
preemptible and sleepable BPF programs will completely remove the need to call
to __bpf_prog_enter/exit().

Detach of a BPF program from the trampoline should not fail. To avoid memory
allocation in detach path the half of the page is used as a reserve and flipped
after each attach/detach. 2k bytes is enough to call 40+ BPF programs directly
which is enough for BPF tracing use cases. This limit can be increased in the
future.

BPF_TRACE_FENTRY programs have access to raw kernel function arguments while
BPF_TRACE_FEXIT programs have access to kernel return value as well. Often
kprobe BPF program remembers function arguments in a map while kretprobe
fetches arguments from a map and analyzes them together with return value.
BPF_TRACE_FEXIT accelerates this typical use case.

Recursion prevention for kprobe BPF programs is done via per-cpu
bpf_prog_active counter. In practice that turned out to be a mistake. It
caused programs to randomly skip execution. The tracing tools missed results
they were looking for. Hence BPF trampoline doesn't provide builtin recursion
prevention. It's a job of BPF program itself and will be addressed in the
follow up patches.

BPF trampoline is intended to be used beyond tracing and fentry/fexit use cases
in the future. For example to remove retpoline cost from XDP programs.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-5-ast@kernel.org

1 files changed, 105 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 8b90db25348a..0d4c5c224d79 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -14,6 +14,8 @@
 #include <linux/numa.h>
 #include <linux/wait.h>
 #include <linux/u64_stats_sync.h>
+#include <linux/refcount.h>
+#include <linux/mutex.h>
 
 struct bpf_verifier_env;
 struct bpf_verifier_log;
@@ -384,6 +386,100 @@ struct bpf_prog_stats {
 	struct u64_stats_sync syncp;
 } __aligned(2 * sizeof(u64));
 
+struct btf_func_model {
+	u8 ret_size;
+	u8 nr_args;
+	u8 arg_size[MAX_BPF_FUNC_ARGS];
+};
+
+/* Restore arguments before returning from trampoline to let original function
+ * continue executing. This flag is used for fentry progs when there are no
+ * fexit progs.
+ */
+#define BPF_TRAMP_F_RESTORE_REGS	BIT(0)
+/* Call original function after fentry progs, but before fexit progs.
+ * Makes sense for fentry/fexit, normal calls and indirect calls.
+ */
+#define BPF_TRAMP_F_CALL_ORIG		BIT(1)
+/* Skip current frame and return to parent.  Makes sense for fentry/fexit
+ * programs only. Should not be used with normal calls and indirect calls.
+ */
+#define BPF_TRAMP_F_SKIP_FRAME		BIT(2)
+
+/* Different use cases for BPF trampoline:
+ * 1. replace nop at the function entry (kprobe equivalent)
+ *    flags = BPF_TRAMP_F_RESTORE_REGS
+ *    fentry = a set of programs to run before returning from trampoline
+ *
+ * 2. replace nop at the function entry (kprobe + kretprobe equivalent)
+ *    flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME
+ *    orig_call = fentry_ip + MCOUNT_INSN_SIZE
+ *    fentry = a set of program to run before calling original function
+ *    fexit = a set of program to run after original function
+ *
+ * 3. replace direct call instruction anywhere in the function body
+ *    or assign a function pointer for indirect call (like tcp_congestion_ops->cong_avoid)
+ *    With flags = 0
+ *      fentry = a set of programs to run before returning from trampoline
+ *    With flags = BPF_TRAMP_F_CALL_ORIG
+ *      orig_call = original callback addr or direct function addr
+ *      fentry = a set of program to run before calling original function
+ *      fexit = a set of program to run after original function
+ */
+int arch_prepare_bpf_trampoline(void *image, struct btf_func_model *m, u32 flags,
+				struct bpf_prog **fentry_progs, int fentry_cnt,
+				struct bpf_prog **fexit_progs, int fexit_cnt,
+				void *orig_call);
+/* these two functions are called from generated trampoline */
+u64 notrace __bpf_prog_enter(void);
+void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start);
+
+enum bpf_tramp_prog_type {
+	BPF_TRAMP_FENTRY,
+	BPF_TRAMP_FEXIT,
+	BPF_TRAMP_MAX
+};
+
+struct bpf_trampoline {
+	/* hlist for trampoline_table */
+	struct hlist_node hlist;
+	/* serializes access to fields of this trampoline */
+	struct mutex mutex;
+	refcount_t refcnt;
+	u64 key;
+	struct {
+		struct btf_func_model model;
+		void *addr;
+	} func;
+	/* list of BPF programs using this trampoline */
+	struct hlist_head progs_hlist[BPF_TRAMP_MAX];
+	/* Number of attached programs. A counter per kind. */
+	int progs_cnt[BPF_TRAMP_MAX];
+	/* Executable image of trampoline */
+	void *image;
+	u64 selector;
+};
+#ifdef CONFIG_BPF_JIT
+struct bpf_trampoline *bpf_trampoline_lookup(u64 key);
+int bpf_trampoline_link_prog(struct bpf_prog *prog);
+int bpf_trampoline_unlink_prog(struct bpf_prog *prog);
+void bpf_trampoline_put(struct bpf_trampoline *tr);
+#else
+static inline struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
+{
+	return NULL;
+}
+static inline int bpf_trampoline_link_prog(struct bpf_prog *prog)
+{
+	return -ENOTSUPP;
+}
+static inline int bpf_trampoline_unlink_prog(struct bpf_prog *prog)
+{
+	return -ENOTSUPP;
+}
+static inline void bpf_trampoline_put(struct bpf_trampoline *tr) {}
+#endif
+
 struct bpf_prog_aux {
 	atomic_t refcnt;
 	u32 used_map_cnt;
@@ -398,6 +494,9 @@ struct bpf_prog_aux {
 	bool verifier_zext; /* Zero extensions has been inserted by verifier. */
 	bool offload_requested;
 	bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
+	enum bpf_tramp_prog_type trampoline_prog_type;
+	struct bpf_trampoline *trampoline;
+	struct hlist_node tramp_hlist;
 	/* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
 	const struct btf_type *attach_func_proto;
 	/* function name for valid attach_btf_id */
@@ -784,6 +883,12 @@ int btf_struct_access(struct bpf_verifier_log *log,
 		      u32 *next_btf_id);
 u32 btf_resolve_helper_id(struct bpf_verifier_log *log, void *, int);
 
+int btf_distill_func_proto(struct bpf_verifier_log *log,
+			   struct btf *btf,
+			   const struct btf_type *func_proto,
+			   const char *func_name,
+			   struct btf_func_model *m);
+
 #else /* !CONFIG_BPF_SYSCALL */
 static inline struct bpf_prog *bpf_prog_get(u32 ufd)
 {