coredump: Retry writes where appropriate

Core dump write operations (especially to a pipe) can be incomplete due
to signal reception or possibly recoverable partial writes.

Previously any incomplete write in the ELF core dumper caused the core
dump to stop, giving short cores in these cases.  Modify the core dumper
to retry the write where appropriate.

Signed-off-by: Paul Smith <paul@mad-scientist.net>
Cc: stable@kernel.org

---

 fs/binfmt_elf.c |   19 ++++++++++++++++++-
 1 files changed, 18 insertions(+), 1 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 40381df..26b03cc 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1119,7 +1119,24 @@ out:
  */
 static int dump_write(struct file *file, const void *addr, int nr)
 {
-	return file->f_op->write(file, addr, nr, &file->f_pos) == nr;
+	const char *p = addr;
+	while (1) {
+		int r = file->f_op->write(file, p, nr, &file->f_pos);
+
+		if (likely(r == nr))
+			return 1;
+
+		if (r == -ERESTARTSYS || r == -EAGAIN || r == -EINTR)
+			/* Ignore signals during coredump. */
+			clear_thread_flag(TIF_SIGPENDING);
+		else if (r > 0) {
+			/* Partial write: try again with the rest. */
+			p += r;
+			nr -= r;
+		} else
+			/* Lose! */
+			return 0;
+	}
 }
 
 static int dump_seek(struct file *file, loff_t off)