Some applications want to prevent the usual "lowest available fd"
allocation from allocating certain file descriptors. For instance, they
may want to prevent allocation of a closed fd 0, 1, or 2 other than via
dup2/dup3, or reserve some low file descriptors for other purposes.
System calls that allocate a specific file descriptor, such as
dup2/dup3, ignore this minimum.
exec resets the minimum fd, to prevent one program from interfering with
another program's expectations about fd allocation.
Test program:
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/prctl.h>
int main(int argc, char *argv[])
{
if (prctl(PR_SET_MIN_FD, 100, 0, 0, 0) < 0)
err(1, "prctl");
int fd = open("/dev/null", O_RDONLY);
if (fd < 0)
err(1, "open");
printf("%d\n", fd); // prints 100
return 0;
}
Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---
fs/file.c | 29 +++++++++++++++++++++++------
include/linux/fdtable.h | 1 +
include/linux/file.h | 2 ++
include/uapi/linux/prctl.h | 4 ++++
kernel/sys.c | 10 ++++++++++
5 files changed, 40 insertions(+), 6 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index a364e1a9b7e8..1b79d4ddedb2 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -286,7 +286,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
spin_lock_init(&newf->file_lock);
newf->resize_in_progress = false;
init_waitqueue_head(&newf->resize_wait);
- newf->next_fd = 0;
new_fdt = &newf->fdtab;
new_fdt->max_fds = NR_OPEN_DEFAULT;
new_fdt->close_on_exec = newf->close_on_exec_init;
@@ -295,6 +294,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
new_fdt->fd = &newf->fd_array[0];
spin_lock(&oldf->file_lock);
+ newf->next_fd = newf->min_fd = oldf->min_fd;
old_fdt = files_fdtable(oldf);
open_files = count_open_files(old_fdt);
@@ -487,9 +487,7 @@ int __alloc_fd(struct files_struct *files,
spin_lock(&files->file_lock);
repeat:
fdt = files_fdtable(files);
- fd = start;
- if (fd < files->next_fd)
- fd = files->next_fd;
+ fd = max3(start, files->min_fd, files->next_fd);
if (fd < fdt->max_fds)
fd = find_next_fd(fdt, fd);
@@ -514,7 +512,7 @@ int __alloc_fd(struct files_struct *files,
goto repeat;
if (start <= files->next_fd)
- files->next_fd = fd + 1;
+ files->next_fd = max(fd + 1, files->min_fd);
__set_open_fd(fd, fdt);
if (flags & O_CLOEXEC)
@@ -550,7 +548,7 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
{
struct fdtable *fdt = files_fdtable(files);
__clear_open_fd(fd, fdt);
- if (fd < files->next_fd)
+ if (fd < files->next_fd && fd >= files->min_fd)
files->next_fd = fd;
}
@@ -679,6 +677,7 @@ void do_close_on_exec(struct files_struct *files)
/* exec unshares first */
spin_lock(&files->file_lock);
+ files->min_fd = 0;
for (i = 0; ; i++) {
unsigned long set;
unsigned fd = i * BITS_PER_LONG;
@@ -860,6 +859,24 @@ bool get_close_on_exec(unsigned int fd)
return res;
}
+void set_min_fd(unsigned int min_fd)
+{
+ struct files_struct *files = current->files;
+ spin_lock(&files->file_lock);
+ files->min_fd = min_fd;
+ spin_unlock(&files->file_lock);
+}
+
+unsigned int get_min_fd(void)
+{
+ struct files_struct *files = current->files;
+ unsigned int min_fd;
+ spin_lock(&files->file_lock);
+ min_fd = files->min_fd;
+ spin_unlock(&files->file_lock);
+ return min_fd;
+}
+
static int do_dup2(struct files_struct *files,
struct file *file, unsigned fd, unsigned flags)
__releases(&files->file_lock)
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index f07c55ea0c22..d1980443d8b3 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -60,6 +60,7 @@ struct files_struct {
*/
spinlock_t file_lock ____cacheline_aligned_in_smp;
unsigned int next_fd;
+ unsigned int min_fd; /* min for "lowest available fd" allocation */
unsigned long close_on_exec_init[1];
unsigned long open_fds_init[1];
unsigned long full_fds_bits_init[1];
diff --git a/include/linux/file.h b/include/linux/file.h
index c6c7b24ea9f7..358202f5951e 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -85,6 +85,8 @@ 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 void set_min_fd(unsigned int min_fd);
+extern unsigned int get_min_fd(void);
extern int get_unused_fd_flags(unsigned flags);
extern void put_unused_fd(unsigned int fd);
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..d0a9ebf46872 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -238,4 +238,8 @@ struct prctl_mm_map {
#define PR_SET_IO_FLUSHER 57
#define PR_GET_IO_FLUSHER 58
+/* Minimum file descriptor for automatic "lowest available fd" allocation */
+#define PR_SET_MIN_FD 59
+#define PR_GET_MIN_FD 60
+
#endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index f9bc5c303e3f..5ab301e97218 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2513,6 +2513,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
break;
+ case PR_SET_MIN_FD:
+ if (arg3 || arg4 || arg5)
+ return -EINVAL;
+ set_min_fd((int)arg2);
+ break;
+ case PR_GET_MIN_FD:
+ if (arg3 || arg4 || arg5)
+ return -EINVAL;
+ error = put_user(get_min_fd(), (int __user *)arg2);
+ break;
default:
error = -EINVAL;
break;
--
2.25.1
