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libbpf: Add bpf_link pinning/unpinning
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With bpf_link abstraction supported by kernel explicitly, add
pinning/unpinning API for links. Also allow to create (open) bpf_link from BPF
FS file.

This API allows to have an "ephemeral" FD-based BPF links (like raw tracepoint
or fexit/freplace attachments) surviving user process exit, by pinning them in
a BPF FS, which is an important use case for long-running BPF programs.

As part of this, expose underlying FD for bpf_link. While legacy bpf_link's
might not have a FD associated with them (which will be expressed as
a bpf_link with fd=-1), kernel's abstraction is based around FD-based usage,
so match it closely. This, subsequently, allows to have a generic
pinning/unpinning API for generalized bpf_link. For some types of bpf_links
kernel might not support pinning, in which case bpf_link__pin() will return
error.

With FD being part of generic bpf_link, also get rid of bpf_link_fd in favor
of using vanialla bpf_link.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200303043159.323675-3-andriin@fb.com
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Andrii Nakryiko authored and Alexei Starovoitov committed Mar 3, 2020
1 parent 70ed506 commit c016b68
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Showing 3 changed files with 114 additions and 27 deletions.
131 changes: 104 additions & 27 deletions tools/lib/bpf/libbpf.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -6931,6 +6931,8 @@ int bpf_prog_load_xattr(const struct bpf_prog_load_attr *attr,
struct bpf_link {
int (*detach)(struct bpf_link *link);
int (*destroy)(struct bpf_link *link);
char *pin_path; /* NULL, if not pinned */
int fd; /* hook FD, -1 if not applicable */
bool disconnected;
};

Expand Down Expand Up @@ -6960,34 +6962,117 @@ int bpf_link__destroy(struct bpf_link *link)
err = link->detach(link);
if (link->destroy)
link->destroy(link);
if (link->pin_path)
free(link->pin_path);
free(link);

return err;
}

struct bpf_link_fd {
struct bpf_link link; /* has to be at the top of struct */
int fd; /* hook FD */
};
int bpf_link__fd(const struct bpf_link *link)
{
return link->fd;
}

const char *bpf_link__pin_path(const struct bpf_link *link)
{
return link->pin_path;
}

static int bpf_link__detach_fd(struct bpf_link *link)
{
return close(link->fd);
}

struct bpf_link *bpf_link__open(const char *path)
{
struct bpf_link *link;
int fd;

fd = bpf_obj_get(path);
if (fd < 0) {
fd = -errno;
pr_warn("failed to open link at %s: %d\n", path, fd);
return ERR_PTR(fd);
}

link = calloc(1, sizeof(*link));
if (!link) {
close(fd);
return ERR_PTR(-ENOMEM);
}
link->detach = &bpf_link__detach_fd;
link->fd = fd;

link->pin_path = strdup(path);
if (!link->pin_path) {
bpf_link__destroy(link);
return ERR_PTR(-ENOMEM);
}

return link;
}

int bpf_link__pin(struct bpf_link *link, const char *path)
{
int err;

if (link->pin_path)
return -EBUSY;
err = make_parent_dir(path);
if (err)
return err;
err = check_path(path);
if (err)
return err;

link->pin_path = strdup(path);
if (!link->pin_path)
return -ENOMEM;

if (bpf_obj_pin(link->fd, link->pin_path)) {
err = -errno;
zfree(&link->pin_path);
return err;
}

pr_debug("link fd=%d: pinned at %s\n", link->fd, link->pin_path);
return 0;
}

int bpf_link__unpin(struct bpf_link *link)
{
int err;

if (!link->pin_path)
return -EINVAL;

err = unlink(link->pin_path);
if (err != 0)
return -errno;

pr_debug("link fd=%d: unpinned from %s\n", link->fd, link->pin_path);
zfree(&link->pin_path);
return 0;
}

static int bpf_link__detach_perf_event(struct bpf_link *link)
{
struct bpf_link_fd *l = (void *)link;
int err;

err = ioctl(l->fd, PERF_EVENT_IOC_DISABLE, 0);
err = ioctl(link->fd, PERF_EVENT_IOC_DISABLE, 0);
if (err)
err = -errno;

close(l->fd);
close(link->fd);
return err;
}

struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
int pfd)
{
char errmsg[STRERR_BUFSIZE];
struct bpf_link_fd *link;
struct bpf_link *link;
int prog_fd, err;

if (pfd < 0) {
Expand All @@ -7005,7 +7090,7 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
link = calloc(1, sizeof(*link));
if (!link)
return ERR_PTR(-ENOMEM);
link->link.detach = &bpf_link__detach_perf_event;
link->detach = &bpf_link__detach_perf_event;
link->fd = pfd;

if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
Expand All @@ -7024,7 +7109,7 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
return ERR_PTR(err);
}
return (struct bpf_link *)link;
return link;
}

/*
Expand Down Expand Up @@ -7312,18 +7397,11 @@ static struct bpf_link *attach_tp(const struct bpf_sec_def *sec,
return link;
}

static int bpf_link__detach_fd(struct bpf_link *link)
{
struct bpf_link_fd *l = (void *)link;

return close(l->fd);
}

struct bpf_link *bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
const char *tp_name)
{
char errmsg[STRERR_BUFSIZE];
struct bpf_link_fd *link;
struct bpf_link *link;
int prog_fd, pfd;

prog_fd = bpf_program__fd(prog);
Expand All @@ -7336,7 +7414,7 @@ struct bpf_link *bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
link = calloc(1, sizeof(*link));
if (!link)
return ERR_PTR(-ENOMEM);
link->link.detach = &bpf_link__detach_fd;
link->detach = &bpf_link__detach_fd;

pfd = bpf_raw_tracepoint_open(tp_name, prog_fd);
if (pfd < 0) {
Expand All @@ -7348,7 +7426,7 @@ struct bpf_link *bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
return ERR_PTR(pfd);
}
link->fd = pfd;
return (struct bpf_link *)link;
return link;
}

static struct bpf_link *attach_raw_tp(const struct bpf_sec_def *sec,
Expand All @@ -7362,7 +7440,7 @@ static struct bpf_link *attach_raw_tp(const struct bpf_sec_def *sec,
struct bpf_link *bpf_program__attach_trace(struct bpf_program *prog)
{
char errmsg[STRERR_BUFSIZE];
struct bpf_link_fd *link;
struct bpf_link *link;
int prog_fd, pfd;

prog_fd = bpf_program__fd(prog);
Expand All @@ -7375,7 +7453,7 @@ struct bpf_link *bpf_program__attach_trace(struct bpf_program *prog)
link = calloc(1, sizeof(*link));
if (!link)
return ERR_PTR(-ENOMEM);
link->link.detach = &bpf_link__detach_fd;
link->detach = &bpf_link__detach_fd;

pfd = bpf_raw_tracepoint_open(NULL, prog_fd);
if (pfd < 0) {
Expand Down Expand Up @@ -7409,10 +7487,9 @@ struct bpf_link *bpf_program__attach(struct bpf_program *prog)

static int bpf_link__detach_struct_ops(struct bpf_link *link)
{
struct bpf_link_fd *l = (void *)link;
__u32 zero = 0;

if (bpf_map_delete_elem(l->fd, &zero))
if (bpf_map_delete_elem(link->fd, &zero))
return -errno;

return 0;
Expand All @@ -7421,7 +7498,7 @@ static int bpf_link__detach_struct_ops(struct bpf_link *link)
struct bpf_link *bpf_map__attach_struct_ops(struct bpf_map *map)
{
struct bpf_struct_ops *st_ops;
struct bpf_link_fd *link;
struct bpf_link *link;
__u32 i, zero = 0;
int err;

Expand Down Expand Up @@ -7453,10 +7530,10 @@ struct bpf_link *bpf_map__attach_struct_ops(struct bpf_map *map)
return ERR_PTR(err);
}

link->link.detach = bpf_link__detach_struct_ops;
link->detach = bpf_link__detach_struct_ops;
link->fd = map->fd;

return (struct bpf_link *)link;
return link;
}

enum bpf_perf_event_ret
Expand Down
5 changes: 5 additions & 0 deletions tools/lib/bpf/libbpf.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -219,6 +219,11 @@ LIBBPF_API void bpf_program__unload(struct bpf_program *prog);

struct bpf_link;

LIBBPF_API struct bpf_link *bpf_link__open(const char *path);
LIBBPF_API int bpf_link__fd(const struct bpf_link *link);
LIBBPF_API const char *bpf_link__pin_path(const struct bpf_link *link);
LIBBPF_API int bpf_link__pin(struct bpf_link *link, const char *path);
LIBBPF_API int bpf_link__unpin(struct bpf_link *link);
LIBBPF_API void bpf_link__disconnect(struct bpf_link *link);
LIBBPF_API int bpf_link__destroy(struct bpf_link *link);

Expand Down
5 changes: 5 additions & 0 deletions tools/lib/bpf/libbpf.map
View file
Original file line number Diff line number Diff line change
Expand Up @@ -238,5 +238,10 @@ LIBBPF_0.0.7 {

LIBBPF_0.0.8 {
global:
bpf_link__fd;
bpf_link__open;
bpf_link__pin;
bpf_link__pin_path;
bpf_link__unpin;
bpf_program__set_attach_target;
} LIBBPF_0.0.7;

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