[PATCH bpf 1/3] bpf: syscall to start at file-descriptor 1

Jesper Dangaard Brouer
2020-06-08T16:51:28Z

This patch change BPF syscall to avoid returning file descriptor value zero.

As mentioned in cover letter, it is very impractical when extending kABI
that the file-descriptor value 'zero' is valid, as this requires new fields
must be initialised as minus-1. First step is to change the kernel such that
BPF-syscall simply doesn't return value zero as a FD number.

This patch achieves this by similar code to anon_inode_getfd(), with the
exception of getting unused FD starting from 1. The kernel already supports
starting from a specific FD value, as this is used by f_dupfd(). It seems
simpler to replicate part of anon_inode_getfd() code and use this start from
offset feature, instead of using f_dupfd() handling afterwards.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
fs/file.c | 2 +-
include/linux/file.h | 1 +
kernel/bpf/syscall.c | 38 ++++++++++++++++++++++++++++++++------
3 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index abb8b7081d7a..122185cb7707 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -535,7 +535,7 @@ int __alloc_fd(struct files_struct *files,
return error;
}

-static int alloc_fd(unsigned start, unsigned flags)
+int alloc_fd(unsigned start, unsigned flags)
{
return __alloc_fd(current->files, start, rlimit(RLIMIT_NOFILE), flags);
}
diff --git a/include/linux/file.h b/include/linux/file.h
index 122f80084a3e..927fb6c2582d 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -85,6 +85,7 @@ extern int f_dupfd(unsigned int from, struct file *file, unsigned flags);
extern int replace_fd(unsigned fd, struct file *file, unsigned flags);
extern void set_close_on_exec(unsigned int fd, int flag);
extern bool get_close_on_exec(unsigned int fd);
+extern int alloc_fd(unsigned start, unsigned flags);
extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile);
extern int get_unused_fd_flags(unsigned flags);
extern void put_unused_fd(unsigned int fd);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4d530b1d5683..6eba236aacd1 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -688,6 +688,32 @@ const struct file_operations bpf_map_fops = {
.poll = bpf_map_poll,
};

+/* Code is similar to anon_inode_getfd(), except starts at FD 1 */
+int bpf_anon_inode_getfd(const char *name, const struct file_operations *fops,
+ void *priv, int flags)
+{
+ int error, fd;
+ struct file *file;
+
+ error = alloc_fd(1, flags);
+ if (error < 0)
+ return error;
+ fd = error;
+
+ file = anon_inode_getfile(name, fops, priv, flags);
+ if (IS_ERR(file)) {
+ error = PTR_ERR(file);
+ goto err_put_unused_fd;
+ }
+ fd_install(fd, file);
+
+ return fd;
+
+err_put_unused_fd:
+ put_unused_fd(fd);
+ return error;
+}
+
int bpf_map_new_fd(struct bpf_map *map, int flags)
{
int ret;
@@ -696,8 +722,8 @@ int bpf_map_new_fd(struct bpf_map *map, int flags)
if (ret < 0)
return ret;

- return anon_inode_getfd("bpf-map", &bpf_map_fops, map,
- flags | O_CLOEXEC);
+ return bpf_anon_inode_getfd("bpf-map", &bpf_map_fops, map,
+ flags | O_CLOEXEC);
}

int bpf_get_file_flag(int flags)
@@ -1840,8 +1866,8 @@ int bpf_prog_new_fd(struct bpf_prog *prog)
if (ret < 0)
return ret;

- return anon_inode_getfd("bpf-prog", &bpf_prog_fops, prog,
- O_RDWR | O_CLOEXEC);
+ return bpf_anon_inode_getfd("bpf-prog", &bpf_prog_fops, prog,
+ O_RDWR | O_CLOEXEC);
}

static struct bpf_prog *____bpf_prog_get(struct fd f)
@@ -2471,7 +2497,7 @@ int bpf_link_settle(struct bpf_link_primer *primer)

int bpf_link_new_fd(struct bpf_link *link)
{
- return anon_inode_getfd("bpf-link", &bpf_link_fops, link, O_CLOEXEC);
+ return bpf_anon_inode_getfd("bpf-link", &bpf_link_fops, link, O_CLOEXEC);
}

struct bpf_link *bpf_link_get_from_fd(u32 ufd)
@@ -4024,7 +4050,7 @@ static int bpf_enable_runtime_stats(void)
return -EBUSY;
}

- fd = anon_inode_getfd("bpf-stats", &bpf_stats_fops, NULL, O_CLOEXEC);
+ fd = bpf_anon_inode_getfd("bpf-stats", &bpf_stats_fops, NULL, O_CLOEXEC);
if (fd >= 0)
static_key_slow_inc(&bpf_stats_enabled_key.key);