bpftool: Probe for memcg-based accounting before bumping rlimit

Bpftool used to bump the memlock rlimit to make sure to be able to load
BPF objects. After the kernel has switched to memcg-based memory
accounting [0] in 5.11, bpftool has relied on libbpf to probe the system
for memcg-based accounting support and for raising the rlimit if
necessary [1]. But this was later reverted, because the probe would
sometimes fail, resulting in bpftool not being able to load all required
objects [2].

Here we add a more efficient probe, in bpftool itself. We first lower
the rlimit to 0, then we attempt to load a BPF object (and finally reset
the rlimit): if the load succeeds, then memcg-based memory accounting is
supported.

This approach was earlier proposed for the probe in libbpf itself [3],
but given that the library may be used in multithreaded applications,
the probe could have undesirable consequences if one thread attempts to
lock kernel memory while memlock rlimit is at 0. Since bpftool is
single-threaded and the rlimit is process-based, this is fine to do in
bpftool itself.

This probe was inspired by the similar one from the cilium/ebpf Go
library [4].

  [0] commit 97306be ("Merge branch 'switch to memcg-based memory accounting'")
  [1] commit a777e18 ("bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK")
  [2] commit 6b4384f ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK"")
  [3] https://lore.kernel.org/bpf/20220609143614.97837-1-quentin@isovalent.com/t/#u
  [4] https://github.com/cilium/ebpf/blob/v0.9.0/rlimit/rlimit.go#L39

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/bpf/20220629111351.47699-1-quentin@isovalent.com

tools/bpf/bpftool/common.c

@@ -13,14 +13,17 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
-#include <linux/limits.h>
-#include <linux/magic.h>
 #include <net/if.h>
 #include <sys/mount.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/vfs.h>
 
+#include <linux/filter.h>
+#include <linux/limits.h>
+#include <linux/magic.h>
+#include <linux/unistd.h>
+
 #include <bpf/bpf.h>
 #include <bpf/hashmap.h>
 #include <bpf/libbpf.h> /* libbpf_num_possible_cpus */
@@ -73,11 +76,73 @@ static bool is_bpffs(char *path)
 	return (unsigned long)st_fs.f_type == BPF_FS_MAGIC;
 }
 
+/* Probe whether kernel switched from memlock-based (RLIMIT_MEMLOCK) to
+ * memcg-based memory accounting for BPF maps and programs. This was done in
+ * commit 97306be45fbe ("Merge branch 'switch to memcg-based memory
+ * accounting'"), in Linux 5.11.
+ *
+ * Libbpf also offers to probe for memcg-based accounting vs rlimit, but does
+ * so by checking for the availability of a given BPF helper and this has
+ * failed on some kernels with backports in the past, see commit 6b4384ff1088
+ * ("Revert "bpftool: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK"").
+ * Instead, we can probe by lowering the process-based rlimit to 0, trying to
+ * load a BPF object, and resetting the rlimit. If the load succeeds then
+ * memcg-based accounting is supported.
+ *
+ * This would be too dangerous to do in the library, because multithreaded
+ * applications might attempt to load items while the rlimit is at 0. Given
+ * that bpftool is single-threaded, this is fine to do here.
+ */
+static bool known_to_need_rlimit(void)
+{
+	struct rlimit rlim_init, rlim_cur_zero = {};
+	struct bpf_insn insns[] = {
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_EXIT_INSN(),
+	};
+	size_t insn_cnt = ARRAY_SIZE(insns);
+	union bpf_attr attr;
+	int prog_fd, err;
+
+	memset(&attr, 0, sizeof(attr));
+	attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
+	attr.insns = ptr_to_u64(insns);
+	attr.insn_cnt = insn_cnt;
+	attr.license = ptr_to_u64("GPL");
+
+	if (getrlimit(RLIMIT_MEMLOCK, &rlim_init))
+		return false;
+
+	/* Drop the soft limit to zero. We maintain the hard limit to its
+	 * current value, because lowering it would be a permanent operation
+	 * for unprivileged users.
+	 */
+	rlim_cur_zero.rlim_max = rlim_init.rlim_max;
+	if (setrlimit(RLIMIT_MEMLOCK, &rlim_cur_zero))
+		return false;
+
+	/* Do not use bpf_prog_load() from libbpf here, because it calls
+	 * bump_rlimit_memlock(), interfering with the current probe.
+	 */
+	prog_fd = syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
+	err = errno;
+
+	/* reset soft rlimit to its initial value */
+	setrlimit(RLIMIT_MEMLOCK, &rlim_init);
+
+	if (prog_fd < 0)
+		return err == EPERM;
+
+	close(prog_fd);
+	return false;
+}
+
 void set_max_rlimit(void)
 {
 	struct rlimit rinf = { RLIM_INFINITY, RLIM_INFINITY };
 
-	setrlimit(RLIMIT_MEMLOCK, &rinf);
+	if (known_to_need_rlimit())
+		setrlimit(RLIMIT_MEMLOCK, &rinf);
 }
 
 static int