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Date: Tue, 23 Jun 2020 13:07:10 +0100
From: Alan Maguire <alan.maguire@...cle.com>
To: ast@...nel.org, daniel@...earbox.net, yhs@...com, andriin@...com,
arnaldo.melo@...il.com
Cc: kafai@...com, songliubraving@...com, john.fastabend@...il.com,
kpsingh@...omium.org, linux@...musvillemoes.dk, joe@...ches.com,
pmladek@...e.com, rostedt@...dmis.org,
sergey.senozhatsky@...il.com, andriy.shevchenko@...ux.intel.com,
corbet@....net, bpf@...r.kernel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH v3 bpf-next 7/8] bpf: add support for %pT format specifier for bpf_trace_printk() helper
Allow %pT[cNx0] format specifier for BTF-based display of data associated
with pointer. The unsafe data modifier 'u' - where the source data
is traversed without copying it to a safe buffer via probe_kernel_read() -
is not supported.
Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
---
include/uapi/linux/bpf.h | 27 ++++++++++++++++++++++-----
kernel/trace/bpf_trace.c | 24 +++++++++++++++++++++++-
tools/include/uapi/linux/bpf.h | 27 ++++++++++++++++++++++-----
3 files changed, 67 insertions(+), 11 deletions(-)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 1968481..ea4fbf3 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -702,7 +702,12 @@ struct bpf_stack_build_id {
* to file *\/sys/kernel/debug/tracing/trace* from DebugFS, if
* available. It can take up to three additional **u64**
* arguments (as an eBPF helpers, the total number of arguments is
- * limited to five).
+ * limited to five), and also supports %pT (BTF-based type
+ * printing), as long as BPF_READ lockdown is not active.
+ * "%pT" takes a "struct __btf_ptr *" as an argument; it
+ * consists of a pointer value and specified BTF type string or id
+ * used to select the type for display. For more details, see
+ * Documentation/core-api/printk-formats.rst.
*
* Each time the helper is called, it appends a line to the trace.
* Lines are discarded while *\/sys/kernel/debug/tracing/trace* is
@@ -738,10 +743,10 @@ struct bpf_stack_build_id {
* The conversion specifiers supported by *fmt* are similar, but
* more limited than for printk(). They are **%d**, **%i**,
* **%u**, **%x**, **%ld**, **%li**, **%lu**, **%lx**, **%lld**,
- * **%lli**, **%llu**, **%llx**, **%p**, **%s**. No modifier (size
- * of field, padding with zeroes, etc.) is available, and the
- * helper will return **-EINVAL** (but print nothing) if it
- * encounters an unknown specifier.
+ * **%lli**, **%llu**, **%llx**, **%p**, **%pT[cNx0], **%s**.
+ * Only %pT supports modifiers, and the helper will return
+ * **-EINVAL** (but print nothing) if it encouters an unknown
+ * specifier.
*
* Also, note that **bpf_trace_printk**\ () is slow, and should
* only be used for debugging purposes. For this reason, a notice
@@ -4260,4 +4265,16 @@ struct bpf_pidns_info {
__u32 pid;
__u32 tgid;
};
+
+/*
+ * struct __btf_ptr is used for %pT (typed pointer) display; the
+ * additional type string/BTF id are used to render the pointer
+ * data as the appropriate type.
+ */
+struct __btf_ptr {
+ void *ptr;
+ const char *type;
+ __u32 id;
+};
+
#endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index e729c9e5..33ddb31 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -374,9 +374,13 @@ static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
}
}
+/* Unsafe BTF display ('u' modifier) is absent here. */
+#define is_btf_safe_modifier(c) \
+ (c == 'c' || c == 'N' || c == 'x' || c == '0')
+
/*
* Only limited trace_printk() conversion specifiers allowed:
- * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %pks %pus %s
+ * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %pks %pus %s %pT
*/
BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
u64, arg2, u64, arg3)
@@ -412,6 +416,24 @@ static void bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
i++;
} else if (fmt[i] == 'p') {
mod[fmt_cnt]++;
+
+ /*
+ * allow BTF type-based printing, but disallow unsafe
+ * mode - this ensures the data is copied safely
+ * using probe_kernel_read() prior to traversing it.
+ */
+ if (fmt[i + 1] == 'T') {
+ int ret;
+
+ ret = security_locked_down(LOCKDOWN_BPF_READ);
+ if (unlikely(ret < 0))
+ return ret;
+ i += 2;
+ while (is_btf_safe_modifier(fmt[i]))
+ i++;
+ goto fmt_next;
+ }
+
if ((fmt[i + 1] == 'k' ||
fmt[i + 1] == 'u') &&
fmt[i + 2] == 's') {
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 1968481..ea4fbf3 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -702,7 +702,12 @@ struct bpf_stack_build_id {
* to file *\/sys/kernel/debug/tracing/trace* from DebugFS, if
* available. It can take up to three additional **u64**
* arguments (as an eBPF helpers, the total number of arguments is
- * limited to five).
+ * limited to five), and also supports %pT (BTF-based type
+ * printing), as long as BPF_READ lockdown is not active.
+ * "%pT" takes a "struct __btf_ptr *" as an argument; it
+ * consists of a pointer value and specified BTF type string or id
+ * used to select the type for display. For more details, see
+ * Documentation/core-api/printk-formats.rst.
*
* Each time the helper is called, it appends a line to the trace.
* Lines are discarded while *\/sys/kernel/debug/tracing/trace* is
@@ -738,10 +743,10 @@ struct bpf_stack_build_id {
* The conversion specifiers supported by *fmt* are similar, but
* more limited than for printk(). They are **%d**, **%i**,
* **%u**, **%x**, **%ld**, **%li**, **%lu**, **%lx**, **%lld**,
- * **%lli**, **%llu**, **%llx**, **%p**, **%s**. No modifier (size
- * of field, padding with zeroes, etc.) is available, and the
- * helper will return **-EINVAL** (but print nothing) if it
- * encounters an unknown specifier.
+ * **%lli**, **%llu**, **%llx**, **%p**, **%pT[cNx0], **%s**.
+ * Only %pT supports modifiers, and the helper will return
+ * **-EINVAL** (but print nothing) if it encouters an unknown
+ * specifier.
*
* Also, note that **bpf_trace_printk**\ () is slow, and should
* only be used for debugging purposes. For this reason, a notice
@@ -4260,4 +4265,16 @@ struct bpf_pidns_info {
__u32 pid;
__u32 tgid;
};
+
+/*
+ * struct __btf_ptr is used for %pT (typed pointer) display; the
+ * additional type string/BTF id are used to render the pointer
+ * data as the appropriate type.
+ */
+struct __btf_ptr {
+ void *ptr;
+ const char *type;
+ __u32 id;
+};
+
#endif /* _UAPI__LINUX_BPF_H__ */
--
1.8.3.1
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