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Subject[PATCH v6 bpf-next 3/6] bpf: add bpf_snprintf_btf helper
Date
A helper is added to support tracing kernel type information in BPF
using the BPF Type Format (BTF). Its signature is

long bpf_snprintf_btf(char *str, u32 str_size, struct btf_ptr *ptr,
u32 btf_ptr_size, u64 flags);

struct btf_ptr * specifies

- a pointer to the data to be traced;
- the BTF id of the type of data pointed to; or
- a string representation of the type of data pointed to
- a flags field is provided for future use; these flags
are not to be confused with the BTF_F_* flags
below that control how the btf_ptr is displayed; the
flags member of the struct btf_ptr may be used to
disambiguate types in kernel versus module BTF, etc;
the main distinction is the flags relate to the type
and information needed in identifying it; not how it
is displayed.

For example a BPF program with a struct sk_buff *skb
could do the following:

static const char skb_type[] = "struct sk_buff";
static struct btf_ptr b = { };

b.ptr = skb;
b.type = skb_type;
bpf_snprintf_btf(str, sizeof(str), &b, sizeof(b), 0, 0);

Default output looks like this:

(struct sk_buff){
.transport_header = (__u16)65535,
.mac_header = (__u16)65535,
.end = (sk_buff_data_t)192,
.head = (unsigned char *)0x000000007524fd8b,
.data = (unsigned char *)0x000000007524fd8b,
.truesize = (unsigned int)768,
.users = (refcount_t){
.refs = (atomic_t){
.counter = (int)1,
},
},
}

Flags modifying display are as follows:

- BTF_F_COMPACT: no formatting around type information
- BTF_F_NONAME: no struct/union member names/types
- BTF_F_PTR_RAW: show raw (unobfuscated) pointer values;
equivalent to %px.
- BTF_F_ZERO: show zero-valued struct/union members;
they are not displayed by default

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
include/linux/bpf.h | 1 +
include/linux/btf.h | 9 ++--
include/uapi/linux/bpf.h | 68 +++++++++++++++++++++++++++
kernel/bpf/core.c | 1 +
kernel/bpf/helpers.c | 4 ++
kernel/trace/bpf_trace.c | 101 +++++++++++++++++++++++++++++++++++++++++
scripts/bpf_helpers_doc.py | 2 +
tools/include/uapi/linux/bpf.h | 68 +++++++++++++++++++++++++++
8 files changed, 250 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 049e50f..a3b40a5 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1795,6 +1795,7 @@ static inline int bpf_fd_reuseport_array_update_elem(struct bpf_map *map,
extern const struct bpf_func_proto bpf_skc_to_tcp_request_sock_proto;
extern const struct bpf_func_proto bpf_skc_to_udp6_sock_proto;
extern const struct bpf_func_proto bpf_copy_from_user_proto;
+extern const struct bpf_func_proto bpf_snprintf_btf_proto;

const struct bpf_func_proto *bpf_tracing_func_proto(
enum bpf_func_id func_id, const struct bpf_prog *prog);
diff --git a/include/linux/btf.h b/include/linux/btf.h
index d0f5d3c..3e5cdc2 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -6,6 +6,7 @@

#include <linux/types.h>
#include <uapi/linux/btf.h>
+#include <uapi/linux/bpf.h>

#define BTF_TYPE_EMIT(type) ((void)(type *)0)

@@ -59,10 +60,10 @@ const struct btf_type *btf_type_id_size(const struct btf *btf,
* - BTF_SHOW_UNSAFE: skip use of bpf_probe_read() to safely read
* data before displaying it.
*/
-#define BTF_SHOW_COMPACT (1ULL << 0)
-#define BTF_SHOW_NONAME (1ULL << 1)
-#define BTF_SHOW_PTR_RAW (1ULL << 2)
-#define BTF_SHOW_ZERO (1ULL << 3)
+#define BTF_SHOW_COMPACT BTF_F_COMPACT
+#define BTF_SHOW_NONAME BTF_F_NONAME
+#define BTF_SHOW_PTR_RAW BTF_F_PTR_RAW
+#define BTF_SHOW_ZERO BTF_F_ZERO
#define BTF_SHOW_UNSAFE (1ULL << 4)

void btf_type_seq_show(const struct btf *btf, u32 type_id, void *obj,
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index a228125..c1675ad 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3586,6 +3586,41 @@ struct bpf_stack_build_id {
* the data in *dst*. This is a wrapper of **copy_from_user**\ ().
* Return
* 0 on success, or a negative error in case of failure.
+ *
+ * long bpf_snprintf_btf(char *str, u32 str_size, struct btf_ptr *ptr, u32 btf_ptr_size, u64 flags)
+ * Description
+ * Use BTF to store a string representation of *ptr*->ptr in *str*,
+ * using *ptr*->type name or *ptr*->type_id. These values should
+ * specify the type *ptr*->ptr points to. Traversing that
+ * data structure using BTF, the type information and values are
+ * stored in the first *str_size* - 1 bytes of *str*. Safe copy of
+ * the pointer data is carried out to avoid kernel crashes during
+ * operation. Smaller types can use string space on the stack;
+ * larger programs can use map data to store the string
+ * representation.
+ *
+ * The string can be subsequently shared with userspace via
+ * bpf_perf_event_output() or ring buffer interfaces.
+ * bpf_trace_printk() is to be avoided as it places too small
+ * a limit on string size to be useful.
+ *
+ * *flags* is a combination of
+ *
+ * **BTF_F_COMPACT**
+ * no formatting around type information
+ * **BTF_F_NONAME**
+ * no struct/union member names/types
+ * **BTF_F_PTR_RAW**
+ * show raw (unobfuscated) pointer values;
+ * equivalent to printk specifier %px.
+ * **BTF_F_ZERO**
+ * show zero-valued struct/union members; they
+ * are not displayed by default
+ *
+ * Return
+ * The number of bytes that were written (or would have been
+ * written if output had to be truncated due to string size),
+ * or a negative error in cases of failure.
*/
#define __BPF_FUNC_MAPPER(FN) \
FN(unspec), \
@@ -3737,6 +3772,7 @@ struct bpf_stack_build_id {
FN(inode_storage_delete), \
FN(d_path), \
FN(copy_from_user), \
+ FN(snprintf_btf), \
/* */

/* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4845,4 +4881,36 @@ struct bpf_sk_lookup {
__u32 local_port; /* Host byte order */
};

+/*
+ * struct btf_ptr is used for typed pointer representation; the
+ * additional type string/BTF type id are used to render the pointer
+ * data as the appropriate type via the bpf_snprintf_btf() helper
+ * above. A flags field - potentially to specify additional details
+ * about the BTF pointer (rather than its mode of display) - is
+ * present for future use. Display flags - BTF_F_* - are
+ * passed to bpf_snprintf_btf separately.
+ */
+struct btf_ptr {
+ void *ptr;
+ const char *type;
+ __u32 type_id;
+ __u32 flags; /* BTF ptr flags; unused at present. */
+};
+
+/*
+ * Flags to control bpf_snprintf_btf() behaviour.
+ * - BTF_F_COMPACT: no formatting around type information
+ * - BTF_F_NONAME: no struct/union member names/types
+ * - BTF_F_PTR_RAW: show raw (unobfuscated) pointer values;
+ * equivalent to %px.
+ * - BTF_F_ZERO: show zero-valued struct/union members; they
+ * are not displayed by default
+ */
+enum {
+ BTF_F_COMPACT = (1ULL << 0),
+ BTF_F_NONAME = (1ULL << 1),
+ BTF_F_PTR_RAW = (1ULL << 2),
+ BTF_F_ZERO = (1ULL << 3),
+};
+
#endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index c4811b13..403fb23 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2216,6 +2216,7 @@ void bpf_user_rnd_init_once(void)
const struct bpf_func_proto bpf_get_current_ancestor_cgroup_id_proto __weak;
const struct bpf_func_proto bpf_get_local_storage_proto __weak;
const struct bpf_func_proto bpf_get_ns_current_pid_tgid_proto __weak;
+const struct bpf_func_proto bpf_snprintf_btf_proto __weak;

const struct bpf_func_proto * __weak bpf_get_trace_printk_proto(void)
{
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 5cc7425..e825441 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -683,6 +683,10 @@ static int __bpf_strtoll(const char *buf, size_t buf_len, u64 flags,
if (!perfmon_capable())
return NULL;
return bpf_get_trace_printk_proto();
+ case BPF_FUNC_snprintf_btf:
+ if (!perfmon_capable())
+ return NULL;
+ return &bpf_snprintf_btf_proto;
case BPF_FUNC_jiffies64:
return &bpf_jiffies64_proto;
default:
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 36508f4..61c274f8 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -7,6 +7,7 @@
#include <linux/slab.h>
#include <linux/bpf.h>
#include <linux/bpf_perf_event.h>
+#include <linux/btf.h>
#include <linux/filter.h>
#include <linux/uaccess.h>
#include <linux/ctype.h>
@@ -16,6 +17,9 @@
#include <linux/error-injection.h>
#include <linux/btf_ids.h>

+#include <uapi/linux/bpf.h>
+#include <uapi/linux/btf.h>
+
#include <asm/tlb.h>

#include "trace_probe.h"
@@ -1147,6 +1151,101 @@ static bool bpf_d_path_allowed(const struct bpf_prog *prog)
.allowed = bpf_d_path_allowed,
};

+#define BTF_F_ALL (BTF_F_COMPACT | BTF_F_NONAME | \
+ BTF_F_PTR_RAW | BTF_F_ZERO)
+
+static int bpf_btf_printf_prepare(struct btf_ptr *ptr, u32 btf_ptr_size,
+ u64 flags, const struct btf **btf,
+ s32 *btf_id)
+{
+ u8 btf_kind = BTF_KIND_TYPEDEF;
+ char type_name[KSYM_NAME_LEN];
+ const struct btf_type *t;
+ const char *btf_type;
+ int ret;
+
+ if (unlikely(flags & ~(BTF_F_ALL)))
+ return -EINVAL;
+
+ if (btf_ptr_size != sizeof(struct btf_ptr))
+ return -EINVAL;
+
+ *btf = bpf_get_btf_vmlinux();
+
+ if (IS_ERR_OR_NULL(*btf))
+ return PTR_ERR(*btf);
+
+ if (ptr->type != NULL) {
+ ret = copy_from_kernel_nofault(type_name, ptr->type,
+ sizeof(type_name));
+ if (ret)
+ return ret;
+
+ btf_type = type_name;
+
+ if (strncmp(btf_type, "struct ", strlen("struct ")) == 0) {
+ btf_kind = BTF_KIND_STRUCT;
+ btf_type += strlen("struct ");
+ } else if (strncmp(btf_type, "union ", strlen("union ")) == 0) {
+ btf_kind = BTF_KIND_UNION;
+ btf_type += strlen("union ");
+ } else if (strncmp(btf_type, "enum ", strlen("enum ")) == 0) {
+ btf_kind = BTF_KIND_ENUM;
+ btf_type += strlen("enum ");
+ }
+
+ if (strlen(btf_type) == 0)
+ return -EINVAL;
+
+ /* Assume type specified is a typedef as there's not much
+ * benefit in specifying int types other than wasting time
+ * on BTF lookups; we optimize for the most useful path.
+ *
+ * Fall back to BTF_KIND_INT if this fails.
+ */
+ *btf_id = btf_find_by_name_kind(*btf, btf_type, btf_kind);
+ if (*btf_id < 0)
+ *btf_id = btf_find_by_name_kind(*btf, btf_type,
+ BTF_KIND_INT);
+ } else if (ptr->type_id > 0)
+ *btf_id = ptr->type_id;
+ else
+ return -EINVAL;
+
+ if (*btf_id > 0)
+ t = btf_type_by_id(*btf, *btf_id);
+ if (*btf_id <= 0 || !t)
+ return -ENOENT;
+
+ return 0;
+}
+
+BPF_CALL_5(bpf_snprintf_btf, char *, str, u32, str_size, struct btf_ptr *, ptr,
+ u32, btf_ptr_size, u64, flags)
+{
+ const struct btf *btf;
+ s32 btf_id;
+ int ret;
+
+ ret = bpf_btf_printf_prepare(ptr, btf_ptr_size, flags, &btf, &btf_id);
+ if (ret)
+ return ret;
+
+ return btf_type_snprintf_show(btf, btf_id, ptr->ptr, str, str_size,
+ flags);
+}
+
+const struct bpf_func_proto bpf_snprintf_btf_proto = {
+ .func = bpf_snprintf_btf,
+ .gpl_only = false,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_PTR_TO_MEM,
+ .arg2_type = ARG_CONST_SIZE,
+ .arg3_type = ARG_PTR_TO_MEM,
+ .arg4_type = ARG_CONST_SIZE,
+ .arg5_type = ARG_ANYTHING,
+};
+
const struct bpf_func_proto *
bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
{
@@ -1233,6 +1332,8 @@ static bool bpf_d_path_allowed(const struct bpf_prog *prog)
return &bpf_get_task_stack_proto;
case BPF_FUNC_copy_from_user:
return prog->aux->sleepable ? &bpf_copy_from_user_proto : NULL;
+ case BPF_FUNC_snprintf_btf:
+ return &bpf_snprintf_btf_proto;
default:
return NULL;
}
diff --git a/scripts/bpf_helpers_doc.py b/scripts/bpf_helpers_doc.py
index 0838817..7d86fdd 100755
--- a/scripts/bpf_helpers_doc.py
+++ b/scripts/bpf_helpers_doc.py
@@ -433,6 +433,7 @@ class PrinterHelpers(Printer):
'struct sk_msg_md',
'struct xdp_md',
'struct path',
+ 'struct btf_ptr',
]
known_types = {
'...',
@@ -474,6 +475,7 @@ class PrinterHelpers(Printer):
'struct udp6_sock',
'struct task_struct',
'struct path',
+ 'struct btf_ptr',
}
mapped_types = {
'u8': '__u8',
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a228125..c1675ad 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3586,6 +3586,41 @@ struct bpf_stack_build_id {
* the data in *dst*. This is a wrapper of **copy_from_user**\ ().
* Return
* 0 on success, or a negative error in case of failure.
+ *
+ * long bpf_snprintf_btf(char *str, u32 str_size, struct btf_ptr *ptr, u32 btf_ptr_size, u64 flags)
+ * Description
+ * Use BTF to store a string representation of *ptr*->ptr in *str*,
+ * using *ptr*->type name or *ptr*->type_id. These values should
+ * specify the type *ptr*->ptr points to. Traversing that
+ * data structure using BTF, the type information and values are
+ * stored in the first *str_size* - 1 bytes of *str*. Safe copy of
+ * the pointer data is carried out to avoid kernel crashes during
+ * operation. Smaller types can use string space on the stack;
+ * larger programs can use map data to store the string
+ * representation.
+ *
+ * The string can be subsequently shared with userspace via
+ * bpf_perf_event_output() or ring buffer interfaces.
+ * bpf_trace_printk() is to be avoided as it places too small
+ * a limit on string size to be useful.
+ *
+ * *flags* is a combination of
+ *
+ * **BTF_F_COMPACT**
+ * no formatting around type information
+ * **BTF_F_NONAME**
+ * no struct/union member names/types
+ * **BTF_F_PTR_RAW**
+ * show raw (unobfuscated) pointer values;
+ * equivalent to printk specifier %px.
+ * **BTF_F_ZERO**
+ * show zero-valued struct/union members; they
+ * are not displayed by default
+ *
+ * Return
+ * The number of bytes that were written (or would have been
+ * written if output had to be truncated due to string size),
+ * or a negative error in cases of failure.
*/
#define __BPF_FUNC_MAPPER(FN) \
FN(unspec), \
@@ -3737,6 +3772,7 @@ struct bpf_stack_build_id {
FN(inode_storage_delete), \
FN(d_path), \
FN(copy_from_user), \
+ FN(snprintf_btf), \
/* */

/* integer value in 'imm' field of BPF_CALL instruction selects which helper
@@ -4845,4 +4881,36 @@ struct bpf_sk_lookup {
__u32 local_port; /* Host byte order */
};

+/*
+ * struct btf_ptr is used for typed pointer representation; the
+ * additional type string/BTF type id are used to render the pointer
+ * data as the appropriate type via the bpf_snprintf_btf() helper
+ * above. A flags field - potentially to specify additional details
+ * about the BTF pointer (rather than its mode of display) - is
+ * present for future use. Display flags - BTF_F_* - are
+ * passed to bpf_snprintf_btf separately.
+ */
+struct btf_ptr {
+ void *ptr;
+ const char *type;
+ __u32 type_id;
+ __u32 flags; /* BTF ptr flags; unused at present. */
+};
+
+/*
+ * Flags to control bpf_snprintf_btf() behaviour.
+ * - BTF_F_COMPACT: no formatting around type information
+ * - BTF_F_NONAME: no struct/union member names/types
+ * - BTF_F_PTR_RAW: show raw (unobfuscated) pointer values;
+ * equivalent to %px.
+ * - BTF_F_ZERO: show zero-valued struct/union members; they
+ * are not displayed by default
+ */
+enum {
+ BTF_F_COMPACT = (1ULL << 0),
+ BTF_F_NONAME = (1ULL << 1),
+ BTF_F_PTR_RAW = (1ULL << 2),
+ BTF_F_ZERO = (1ULL << 3),
+};
+
#endif /* _UAPI__LINUX_BPF_H__ */
--
1.8.3.1
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