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Subject[PATCH 5/6] exec: Move handling of the point of no return to the top level

Move the handing of the point of no return from search_binary_handler
into __do_execve_file so that it is easier to find, and to keep
things robust in the face of change.

Make it clear that an existing fatal signal will take precedence over
a forced SIGSEGV by not forcing SIGSEGV if a fatal signal is already
pending. This does not change the behavior but it saves a reader
of the code the tedium of reading and understanding force_sig
and the signal delivery code.

Update the comment in begin_new_exec about where SIGSEGV is forced.

Keep point_of_no_return from being a mystery by documenting
what the code is doing where it forces SIGSEGV if the
code is past the point of no return.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/exec.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 15682a1dfee9..443eb960f9a0 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1329,8 +1329,8 @@ int begin_new_exec(struct linux_binprm * bprm)
/*
* With the new mm installed it is completely impossible to
* fail and return to the original process. If anything from
- * here on returns an error, the check in
- * search_binary_handler() will SEGV current.
+ * here on returns an error, the check in __do_execve_file()
+ * will SEGV current.
*/
bprm->point_of_no_return = true;
bprm->mm = NULL;
@@ -1722,13 +1722,8 @@ int search_binary_handler(struct linux_binprm *bprm)

read_lock(&binfmt_lock);
put_binfmt(fmt);
- if (retval < 0 && bprm->point_of_no_return) {
- /* we got to flush_old_exec() and failed after it */
- read_unlock(&binfmt_lock);
- force_sigsegv(SIGSEGV);
- return retval;
- }
- if (retval != -ENOEXEC || !bprm->file) {
+ if (bprm->point_of_no_return || !bprm->file ||
+ (retval != -ENOEXEC)) {
read_unlock(&binfmt_lock);
return retval;
}
@@ -1899,6 +1894,14 @@ static int __do_execve_file(int fd, struct filename *filename,
return retval;

out:
+ /*
+ * If past the point of no return ensure the the code never
+ * returns to the userspace process. Use an existing fatal
+ * signal if present otherwise terminate the process with
+ * SIGSEGV.
+ */
+ if (bprm->point_of_no_return && !fatal_signal_pending(current))
+ force_sigsegv(SIGSEGV);
if (bprm->mm) {
acct_arg_size(bprm, 0);
mmput(bprm->mm);
--
2.20.1
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