bpf: Support replacing cgroup-bpf program in MULTI mode

The common use-case in production is to have multiple cgroup-bpf
programs per attach type that cover multiple use-cases. Such programs
are attached with BPF_F_ALLOW_MULTI and can be maintained by different
people.

Order of programs usually matters, for example imagine two egress
programs: the first one drops packets and the second one counts packets.
If they're swapped the result of counting program will be different.

It brings operational challenges with updating cgroup-bpf program(s)
attached with BPF_F_ALLOW_MULTI since there is no way to replace a
program:

* One way to update is to detach all programs first and then attach the
  new version(s) again in the right order. This introduces an
  interruption in the work a program is doing and may not be acceptable
  (e.g. if it's egress firewall);

* Another way is attach the new version of a program first and only then
  detach the old version. This introduces the time interval when two
  versions of same program are working, what may not be acceptable if a
  program is not idempotent. It also imposes additional burden on
  program developers to make sure that two versions of their program can
  co-exist.

Solve the problem by introducing a "replace" mode in BPF_PROG_ATTACH
command for cgroup-bpf programs being attached with BPF_F_ALLOW_MULTI
flag. This mode is enabled by newly introduced BPF_F_REPLACE attach flag
and bpf_attr.replace_bpf_fd attribute to pass fd of the old program to
replace

That way user can replace any program among those attached with
BPF_F_ALLOW_MULTI flag without the problems described above.

Details of the new API:

* If BPF_F_REPLACE is set but replace_bpf_fd doesn't have valid
  descriptor of BPF program, BPF_PROG_ATTACH will return corresponding
  error (EINVAL or EBADF).

* If replace_bpf_fd has valid descriptor of BPF program but such a
  program is not attached to specified cgroup, BPF_PROG_ATTACH will
  return ENOENT.

BPF_F_REPLACE is introduced to make the user intent clear, since
replace_bpf_fd alone can't be used for this (its default value, 0, is a
valid fd). BPF_F_REPLACE also makes it possible to extend the API in the
future (e.g. add BPF_F_BEFORE and BPF_F_AFTER if needed).

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrii Narkyiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/30cd850044a0057bdfcaaf154b7d2f39850ba813.1576741281.git.rdna@fb.com

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index 169fd25f6bc2d697f078abd3e9e430499fdb9aa7..18f6a6da7c3c84919101c0bcfafece640a551ed2 100644 (file)
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -85,6 +85,7 @@ int cgroup_bpf_inherit(struct cgroup *cgrp);
 void cgroup_bpf_offline(struct cgroup *cgrp);
 
 int __cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
+                       struct bpf_prog *replace_prog,
                        enum bpf_attach_type type, u32 flags);
 int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
                        enum bpf_attach_type type);
@@ -93,7 +94,8 @@ int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 
 /* Wrapper for __cgroup_bpf_*() protected by cgroup_mutex */
 int cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
-                     enum bpf_attach_type type, u32 flags);
+                     struct bpf_prog *replace_prog, enum bpf_attach_type type,
+                     u32 flags);
 int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
                      enum bpf_attach_type type, u32 flags);
 int cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index dbbcf0b02970be868446632329d078ad3b51f9a1..7df436da542d76ddd73eb9a83851071e6e2f4fe3 100644 (file)
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -231,6 +231,11 @@ enum bpf_attach_type {
  * When children program makes decision (like picking TCP CA or sock bind)
  * parent program has a chance to override it.
  *
+ * With BPF_F_ALLOW_MULTI a new program is added to the end of the list of
+ * programs for a cgroup. Though it's possible to replace an old program at
+ * any position by also specifying BPF_F_REPLACE flag and position itself in
+ * replace_bpf_fd attribute. Old program at this position will be released.
+ *
  * A cgroup with MULTI or OVERRIDE flag allows any attach flags in sub-cgroups.
  * A cgroup with NONE doesn't allow any programs in sub-cgroups.
  * Ex1:
@@ -249,6 +254,7 @@ enum bpf_attach_type {
  */
 #define BPF_F_ALLOW_OVERRIDE   (1U << 0)
 #define BPF_F_ALLOW_MULTI      (1U << 1)
+#define BPF_F_REPLACE          (1U << 2)
 
 /* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
  * verifier will perform strict alignment checking as if the kernel
@@ -442,6 +448,10 @@ union bpf_attr {
                __u32           attach_bpf_fd;  /* eBPF program to attach */
                __u32           attach_type;
                __u32           attach_flags;
+               __u32           replace_bpf_fd; /* previously attached eBPF
+                                                * program to replace if
+                                                * BPF_F_REPLACE is used
+                                                */
        };
 
        struct { /* anonymous struct used by BPF_PROG_TEST_RUN command */

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 283efe3ce052a52fdc300dcb559aa434f6237dfe..45346c79613a97f190123308063a92402411646d 100644 (file)
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -282,14 +282,17 @@ static int update_effective_progs(struct cgroup *cgrp,
  *                         propagate the change to descendants
  * @cgrp: The cgroup which descendants to traverse
  * @prog: A program to attach
+ * @replace_prog: Previously attached program to replace if BPF_F_REPLACE is set
  * @type: Type of attach operation
  * @flags: Option flags
  *
  * Must be called with cgroup_mutex held.
  */
 int __cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
+                       struct bpf_prog *replace_prog,
                        enum bpf_attach_type type, u32 flags)
 {
+       u32 saved_flags = (flags & (BPF_F_ALLOW_OVERRIDE | BPF_F_ALLOW_MULTI));
        struct list_head *progs = &cgrp->bpf.progs[type];
        struct bpf_prog *old_prog = NULL;
        struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE],
@@ -298,14 +301,15 @@ int __cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
        enum bpf_cgroup_storage_type stype;
        int err;
 
-       if ((flags & BPF_F_ALLOW_OVERRIDE) && (flags & BPF_F_ALLOW_MULTI))
+       if (((flags & BPF_F_ALLOW_OVERRIDE) && (flags & BPF_F_ALLOW_MULTI)) ||
+           ((flags & BPF_F_REPLACE) && !(flags & BPF_F_ALLOW_MULTI)))
                /* invalid combination */
                return -EINVAL;
 
        if (!hierarchy_allows_attach(cgrp, type))
                return -EPERM;
 
-       if (!list_empty(progs) && cgrp->bpf.flags[type] != flags)
+       if (!list_empty(progs) && cgrp->bpf.flags[type] != saved_flags)
                /* Disallow attaching non-overridable on top
                 * of existing overridable in this cgroup.
                 * Disallow attaching multi-prog if overridable or none
@@ -320,7 +324,12 @@ int __cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
                        if (pl->prog == prog)
                                /* disallow attaching the same prog twice */
                                return -EINVAL;
+                       if (pl->prog == replace_prog)
+                               replace_pl = pl;
                }
+               if ((flags & BPF_F_REPLACE) && !replace_pl)
+                       /* prog to replace not found for cgroup */
+                       return -ENOENT;
        } else if (!list_empty(progs)) {
                replace_pl = list_first_entry(progs, typeof(*pl), node);
        }
@@ -356,7 +365,7 @@ int __cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
        for_each_cgroup_storage_type(stype)
                pl->storage[stype] = storage[stype];
 
-       cgrp->bpf.flags[type] = flags;
+       cgrp->bpf.flags[type] = saved_flags;
 
        err = update_effective_progs(cgrp, type);
        if (err)
@@ -522,6 +531,7 @@ int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 int cgroup_bpf_prog_attach(const union bpf_attr *attr,
                           enum bpf_prog_type ptype, struct bpf_prog *prog)
 {
+       struct bpf_prog *replace_prog = NULL;
        struct cgroup *cgrp;
        int ret;
 
@@ -529,8 +539,20 @@ int cgroup_bpf_prog_attach(const union bpf_attr *attr,
        if (IS_ERR(cgrp))
                return PTR_ERR(cgrp);
 
-       ret = cgroup_bpf_attach(cgrp, prog, attr->attach_type,
+       if ((attr->attach_flags & BPF_F_ALLOW_MULTI) &&
+           (attr->attach_flags & BPF_F_REPLACE)) {
+               replace_prog = bpf_prog_get_type(attr->replace_bpf_fd, ptype);
+               if (IS_ERR(replace_prog)) {
+                       cgroup_put(cgrp);
+                       return PTR_ERR(replace_prog);
+               }
+       }
+
+       ret = cgroup_bpf_attach(cgrp, prog, replace_prog, attr->attach_type,
                                attr->attach_flags);
+
+       if (replace_prog)
+               bpf_prog_put(replace_prog);
        cgroup_put(cgrp);
        return ret;
 }

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index b08c362f4e020751e999c60da8c9794d889c52d2..81ee8595dfee9cc5c1de24e0b6e5a5558c088917 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2073,10 +2073,10 @@ static int bpf_prog_attach_check_attach_type(const struct bpf_prog *prog,
        }
 }
 
-#define BPF_PROG_ATTACH_LAST_FIELD attach_flags
+#define BPF_PROG_ATTACH_LAST_FIELD replace_bpf_fd
 
 #define BPF_F_ATTACH_MASK \
-       (BPF_F_ALLOW_OVERRIDE | BPF_F_ALLOW_MULTI)
+       (BPF_F_ALLOW_OVERRIDE | BPF_F_ALLOW_MULTI | BPF_F_REPLACE)
 
 static int bpf_prog_attach(const union bpf_attr *attr)
 {

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 735af8f15f95406d8d468d2fbacd3ff7ed2a715e..725365df066d7175715c3777db6a8fbb1cdf5cdf 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6288,12 +6288,13 @@ void cgroup_sk_free(struct sock_cgroup_data *skcd)
 
 #ifdef CONFIG_CGROUP_BPF
 int cgroup_bpf_attach(struct cgroup *cgrp, struct bpf_prog *prog,
-                     enum bpf_attach_type type, u32 flags)
+                     struct bpf_prog *replace_prog, enum bpf_attach_type type,
+                     u32 flags)
 {
        int ret;
 
        mutex_lock(&cgroup_mutex);
-       ret = __cgroup_bpf_attach(cgrp, prog, type, flags);
+       ret = __cgroup_bpf_attach(cgrp, prog, replace_prog, type, flags);
        mutex_unlock(&cgroup_mutex);
        return ret;
 }

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index dbbcf0b02970be868446632329d078ad3b51f9a1..7df436da542d76ddd73eb9a83851071e6e2f4fe3 100644 (file)
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -231,6 +231,11 @@ enum bpf_attach_type {
  * When children program makes decision (like picking TCP CA or sock bind)
  * parent program has a chance to override it.
  *
+ * With BPF_F_ALLOW_MULTI a new program is added to the end of the list of
+ * programs for a cgroup. Though it's possible to replace an old program at
+ * any position by also specifying BPF_F_REPLACE flag and position itself in
+ * replace_bpf_fd attribute. Old program at this position will be released.
+ *
  * A cgroup with MULTI or OVERRIDE flag allows any attach flags in sub-cgroups.
  * A cgroup with NONE doesn't allow any programs in sub-cgroups.
  * Ex1:
@@ -249,6 +254,7 @@ enum bpf_attach_type {
  */
 #define BPF_F_ALLOW_OVERRIDE   (1U << 0)
 #define BPF_F_ALLOW_MULTI      (1U << 1)
+#define BPF_F_REPLACE          (1U << 2)
 
 /* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
  * verifier will perform strict alignment checking as if the kernel
@@ -442,6 +448,10 @@ union bpf_attr {
                __u32           attach_bpf_fd;  /* eBPF program to attach */
                __u32           attach_type;
                __u32           attach_flags;
+               __u32           replace_bpf_fd; /* previously attached eBPF
+                                                * program to replace if
+                                                * BPF_F_REPLACE is used
+                                                */
        };
 
        struct { /* anonymous struct used by BPF_PROG_TEST_RUN command */