Merge branch 'setsockopt-extra-mem'

Stanislav Fomichev says:

====================
Current setsockopt hook is limited to the size of the buffer that
user had supplied. Since we always allocate memory and copy the value
into kernel space, allocate just a little bit more in case BPF
program needs to override input data with a larger value.

The canonical example is TCP_CONGESTION socket option where
input buffer is a string and if user calls it with a short string,
BPF program has no way of extending it.

The tests are extended with TCP_CONGESTION use case.
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/cgroup.c

@@ -964,7 +964,6 @@ static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
 		return -ENOMEM;
 
 	ctx->optval_end = ctx->optval + max_optlen;
-	ctx->optlen = max_optlen;
 
 	return 0;
 }
@@ -984,7 +983,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 		.level = *level,
 		.optname = *optname,
 	};
-	int ret;
+	int ret, max_optlen;
 
 	/* Opportunistic check to see whether we have any BPF program
 	 * attached to the hook so we don't waste time allocating
@@ -994,10 +993,18 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 	    __cgroup_bpf_prog_array_is_empty(cgrp, BPF_CGROUP_SETSOCKOPT))
 		return 0;
 
-	ret = sockopt_alloc_buf(&ctx, *optlen);
+	/* Allocate a bit more than the initial user buffer for
+	 * BPF program. The canonical use case is overriding
+	 * TCP_CONGESTION(nv) to TCP_CONGESTION(cubic).
+	 */
+	max_optlen = max_t(int, 16, *optlen);
+
+	ret = sockopt_alloc_buf(&ctx, max_optlen);
 	if (ret)
 		return ret;
 
+	ctx.optlen = *optlen;
+
 	if (copy_from_user(ctx.optval, optval, *optlen) != 0) {
 		ret = -EFAULT;
 		goto out;
@@ -1016,7 +1023,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 	if (ctx.optlen == -1) {
 		/* optlen set to -1, bypass kernel */
 		ret = 1;
-	} else if (ctx.optlen > *optlen || ctx.optlen < -1) {
+	} else if (ctx.optlen > max_optlen || ctx.optlen < -1) {
 		/* optlen is out of bounds */
 		ret = -EFAULT;
 	} else {
@@ -1063,6 +1070,8 @@ int __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
 	if (ret)
 		return ret;
 
+	ctx.optlen = max_optlen;
+
 	if (!retval) {
 		/* If kernel getsockopt finished successfully,
 		 * copy whatever was returned to the user back

tools/testing/selftests/bpf/progs/sockopt_sk.c

 // SPDX-License-Identifier: GPL-2.0
+#include <string.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <linux/bpf.h>
 #include "bpf_helpers.h"
 
@@ -42,6 +44,14 @@ int _getsockopt(struct bpf_sockopt *ctx)
 		return 1;
 	}
 
+	if (ctx->level == SOL_TCP && ctx->optname == TCP_CONGESTION) {
+		/* Not interested in SOL_TCP:TCP_CONGESTION;
+		 * let next BPF program in the cgroup chain or kernel
+		 * handle it.
+		 */
+		return 1;
+	}
+
 	if (ctx->level != SOL_CUSTOM)
 		return 0; /* EPERM, deny everything except custom level */
 
@@ -91,6 +101,18 @@ int _setsockopt(struct bpf_sockopt *ctx)
 		return 1;
 	}
 
+	if (ctx->level == SOL_TCP && ctx->optname == TCP_CONGESTION) {
+		/* Always use cubic */
+
+		if (optval + 5 > optval_end)
+			return 0; /* EPERM, bounds check */
+
+		memcpy(optval, "cubic", 5);
+		ctx->optlen = 5;
+
+		return 1;
+	}
+
 	if (ctx->level != SOL_CUSTOM)
 		return 0; /* EPERM, deny everything except custom level */
 

tools/testing/selftests/bpf/test_sockopt_sk.c

@@ -6,6 +6,7 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 
 #include <linux/filter.h>
 #include <bpf/bpf.h>
@@ -25,6 +26,7 @@ static int getsetsockopt(void)
 	union {
 		char u8[4];
 		__u32 u32;
+		char cc[16]; /* TCP_CA_NAME_MAX */
 	} buf = {};
 	socklen_t optlen;
 
@@ -115,6 +117,29 @@ static int getsetsockopt(void)
 		goto err;
 	}
 
+	/* TCP_CONGESTION can extend the string */
+
+	strcpy(buf.cc, "nv");
+	err = setsockopt(fd, SOL_TCP, TCP_CONGESTION, &buf, strlen("nv"));
+	if (err) {
+		log_err("Failed to call setsockopt(TCP_CONGESTION)");
+		goto err;
+	}
+
+
+	optlen = sizeof(buf.cc);
+	err = getsockopt(fd, SOL_TCP, TCP_CONGESTION, &buf, &optlen);
+	if (err) {
+		log_err("Failed to call getsockopt(TCP_CONGESTION)");
+		goto err;
+	}
+
+	if (strcmp(buf.cc, "cubic") != 0) {
+		log_err("Unexpected getsockopt(TCP_CONGESTION) %s != %s",
+			buf.cc, "cubic");
+		goto err;
+	}
+
 	close(fd);
 	return 0;
 err: