netdev - [PATCH bpf-next] tools: bpftool: allow unprivileged users to probe features

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Date:   Thu, 23 Apr 2020 17:04:55 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Quentin Monnet <quentin@...valent.com>,
        Richard Palethorpe <rpalethorpe@...e.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: [PATCH bpf-next] tools: bpftool: allow unprivileged users to probe features

There is demand for a way to identify what BPF helper functions are
available to unprivileged users. To do so, allow unprivileged users to
run "bpftool feature probe" to list BPF-related features. This will only
show features accessible to those users, and may not reflect the full
list of features available (to administrators) on the system. For
non-JSON output, print an informational message stating so at the top of
the list.

Note that there is no particular reason why the probes were restricted
to root, other than the fact I did not need them for unprivileged and
did not bother with the additional checks at the time probes were added.

Cc: Richard Palethorpe <rpalethorpe@...e.com>
Cc: Michael Kerrisk <mtk.manpages@...il.com>
Signed-off-by: Quentin Monnet <quentin@...valent.com>
---
 .../bpftool/Documentation/bpftool-feature.rst |  4 +++
 tools/bpf/bpftool/feature.c                   | 32 +++++++++++++------
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-feature.rst b/tools/bpf/bpftool/Documentation/bpftool-feature.rst
index b04156cfd7a3..313888e87249 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-feature.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-feature.rst
@@ -49,6 +49,10 @@ DESCRIPTION
 		  Keyword **kernel** can be omitted. If no probe target is
 		  specified, probing the kernel is the default behaviour.
 
+		  Running this command as an unprivileged user will dump only
+		  the features available to the user, which usually represent a
+		  small subset of the parameters supported by the system.
+
 	**bpftool feature probe dev** *NAME* [**full**] [**macros** [**prefix** *PREFIX*]]
 		  Probe network device for supported eBPF features and dump
 		  results to the console.
diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
index 88718ee6a438..f455bc5fcc64 100644
--- a/tools/bpf/bpftool/feature.c
+++ b/tools/bpf/bpftool/feature.c
@@ -471,6 +471,11 @@ probe_prog_type(enum bpf_prog_type prog_type, bool *supported_types,
 		}
 
 	res = bpf_probe_prog_type(prog_type, ifindex);
+	/* Probe may succeed even if program load fails, for unprivileged users
+	 * check that we did not fail because of insufficient permissions
+	 */
+	if (geteuid() && errno == EPERM)
+		res = false;
 
 	supported_types[prog_type] |= res;
 
@@ -499,6 +504,10 @@ probe_map_type(enum bpf_map_type map_type, const char *define_prefix,
 
 	res = bpf_probe_map_type(map_type, ifindex);
 
+	/* Probe result depends on the success of map creation, no additional
+	 * check required for unprivileged users
+	 */
+
 	maxlen = sizeof(plain_desc) - strlen(plain_comment) - 1;
 	if (strlen(map_type_name[map_type]) > maxlen) {
 		p_info("map type name too long");
@@ -518,12 +527,17 @@ probe_helper_for_progtype(enum bpf_prog_type prog_type, bool supported_type,
 			  const char *define_prefix, unsigned int id,
 			  const char *ptype_name, __u32 ifindex)
 {
-	bool res;
+	bool res = false;
 
-	if (!supported_type)
-		res = false;
-	else
+	if (supported_type) {
 		res = bpf_probe_helper(id, prog_type, ifindex);
+		/* Probe may succeed even if program load fails, for
+		 * unprivileged users check that we did not fail because of
+		 * insufficient permissions
+		 */
+		if (geteuid() && errno == EPERM)
+			res = false;
+	}
 
 	if (json_output) {
 		if (res)
@@ -729,13 +743,11 @@ static int do_probe(int argc, char **argv)
 	__u32 ifindex = 0;
 	char *ifname;
 
-	/* Detection assumes user has sufficient privileges (CAP_SYS_ADMIN).
-	 * Let's approximate, and restrict usage to root user only.
+	/* Full feature detection requires CAP_SYS_ADMIN privilege.
+	 * Let's approximate, and warn if user is not root.
 	 */
-	if (geteuid()) {
-		p_err("please run this command as root user");
-		return -1;
-	}
+	if (geteuid())
+		p_info("probing as unprivileged user, run as root to see all system features");
 
 	set_max_rlimit();
 
-- 
2.20.1