[PATCH v7 5/7] mm: support both pid and pidfd for process_madvise

[Minchan Kim]

There is a demand[1] to support pid as well pidfd for process_madvise
to reduce unnecessary syscall to get pidfd if the user has control of
the target process(ie, they could guarantee the process is not gone
or pid is not reused).

This patch aims for supporting both options like waitid(2). So, the
syscall is currently,

	int process_madvise(int which, pid_t pid, void *addr,
		size_t length, int advise, unsigned long flag);

@which is actually idtype_t for userspace libray and currently,
it supports P_PID and P_PIDFD.

[1]  https://lore.kernel.org/linux-mm/9d849087-3359-c4ab-fbec-859e8186c509@xxxxxxxxxxxxx/

Cc: Christian Brauner <christian@xxxxxxxxxx>
Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Suggested-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 include/linux/syscalls.h |  3 ++-
 mm/madvise.c             | 34 ++++++++++++++++++++++------------
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index e4cd2c2f8bb4..f5ada20e2943 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -876,7 +876,8 @@ asmlinkage long sys_munlockall(void);
 asmlinkage long sys_mincore(unsigned long start, size_t len,
 				unsigned char __user * vec);
 asmlinkage long sys_madvise(unsigned long start, size_t len, int behavior);
-asmlinkage long sys_process_madvise(int pidfd, unsigned long start,
+
+asmlinkage long sys_process_madvise(int which, pid_t pid, unsigned long start,
 			size_t len, int behavior, unsigned long flags);
 asmlinkage long sys_remap_file_pages(unsigned long start, unsigned long size,
 			unsigned long prot, unsigned long pgoff,
diff --git a/mm/madvise.c b/mm/madvise.c
index 6543f2bfc3d8..e794367f681e 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1182,11 +1182,10 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
 	return do_madvise(current, current->mm, start, len_in, behavior);
 }
 
-SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
+SYSCALL_DEFINE6(process_madvise, int, which, pid_t, upid, unsigned long, start,
 		size_t, len_in, int, behavior, unsigned long, flags)
 {
 	int ret;
-	struct fd f;
 	struct pid *pid;
 	struct task_struct *task;
 	struct mm_struct *mm;
@@ -1197,20 +1196,31 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
 	if (!process_madvise_behavior_valid(behavior))
 		return -EINVAL;
 
-	f = fdget(pidfd);
-	if (!f.file)
-		return -EBADF;
+	switch (which) {
+	case P_PID:
+		if (upid <= 0)
+			return -EINVAL;
+
+		pid = find_get_pid(upid);
+		if (!pid)
+			return -ESRCH;
+		break;
+	case P_PIDFD:
+		if (upid < 0)
+			return -EINVAL;
 
-	pid = pidfd_pid(f.file);
-	if (IS_ERR(pid)) {
-		ret = PTR_ERR(pid);
-		goto fdput;
+		pid = pidfd_get_pid(upid);
+		if (IS_ERR(pid))
+			return PTR_ERR(pid);
+		break;
+	default:
+		return -EINVAL;
 	}
 
 	task = get_pid_task(pid, PIDTYPE_PID);
 	if (!task) {
 		ret = -ESRCH;
-		goto fdput;
+		goto put_pid;
 	}
 
 	mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
@@ -1223,7 +1233,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
 	mmput(mm);
 release_task:
 	put_task_struct(task);
-fdput:
-	fdput(f);
+put_pid:
+	put_pid(pid);
 	return ret;
 }
-- 
2.25.0.265.gbab2e86ba0-goog