From: David Herrmann <dh.herrmann@gmail.com>
Date: Mon, 18 Aug 2014 11:05:48 +0000 (+0200)
Subject: memfd: map unsealed files as MAP_SHARED
X-Git-Tag: v216~61^2~3
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?a=commitdiff_plain;h=23972f4244f7609658c2a17f85508d50e4739990;p=elogind.git

memfd: map unsealed files as MAP_SHARED

We need to map sealed files as MAP_PRIVATE so far as the kernel treats
MAP_SHARED as writable mapping (you can run mprotect(PROT_WRITE) at any
time on those). However, unsealed files must be mapped as MAP_SHARED.
Otherwise, we never end up writing to the real file.
---

diff --git a/src/shared/memfd.c b/src/shared/memfd.c
index 6804b4236..d94c6268d 100644
--- a/src/shared/memfd.c
+++ b/src/shared/memfd.c
@@ -176,7 +176,11 @@ int sd_memfd_map(sd_memfd *m, uint64_t offset, size_t size, void **p) {
         if (sealed < 0)
                 return sealed;
 
-        q = mmap(NULL, size, sealed ? PROT_READ : PROT_READ|PROT_WRITE, MAP_PRIVATE, m->fd, offset);
+        if (sealed)
+                q = mmap(NULL, size, PROT_READ, MAP_PRIVATE, m->fd, offset);
+        else
+                q = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, m->fd, offset);
+
         if (q == MAP_FAILED)
                 return -errno;