[lxc-devel] [lxc/master] cgfsng: fix cgroup attach cgroup creation

[brauner on Github]

The following pull request was submitted through Github.
It can be accessed and reviewed at: https://github.com/lxc/lxc/pull/3526

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=== Description (from pull-request) ===
```
cgroups/cgfsng.c: In function ‘cgroup_attach_leaf.constprop’:
cgroups/cgfsng.c:2221:10: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 2221 |   *slash = '\0';
      |   ~~~~~~~^~~~~~
cgroups/cgfsng.c:2213:8: note: at offset -13 to object ‘attach_cgroup’ with size 23 declared here
 2213 |   char attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        ^~~~~~~~~~~~~
cgroups/cgfsng.c:2229:10: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 2229 |   *slash = '/';
      |   ~~~~~~~^~~~~
cgroups/cgfsng.c:2213:8: note: at offset -13 to object ‘attach_cgroup’ with size 23 declared here
 2213 |   char attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        ^~~~~~~~~~~~~
cgroups/cgfsng.c:2229:10: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 2229 |   *slash = '/';
      |   ~~~~~~~^~~~~
cgroups/cgfsng.c:2213:8: note: at offset -13 to object ‘attach_cgroup’ with size 23 declared here
 2213 |   char attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        ^~~~~~~~~~~~~
```

Link: https://launchpadlibrarian.net/494354168/buildlog_ubuntu-groovy-armhf.lxc_1%3A4.0.4-0ubuntu1_BUILDING.txt.gz
Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com>

From c80c9a70bcad9da9fe56f23ed971d74e4df19a95 Mon Sep 17 00:00:00 2001
From: Christian Brauner <christian.brau...@ubuntu.com>
Date: Fri, 21 Aug 2020 09:59:18 +0200
Subject: [PATCH] cgfsng: fix cgroup attach cgroup creation
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

cgroups/cgfsng.c: In function 
‘cgroup_attach_leaf.constprop’:
cgroups/cgfsng.c:2221:10: error: writing 1 byte 
into a region of size 0 [-Werror=stringop-overflow=]
 2221 |   *slash = '\0';
      |   ~~~~~~~^~~~~~
cgroups/cgfsng.c:2213:8: note: at offset -13 to 
object ‘attach_cgroup’ with size 23 declared here
 2213 |   char 
attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        ^~~~~~~~~~~~~
cgroups/cgfsng.c:2229:10: error: writing 1 byte 
into a region of size 0 [-Werror=stringop-overflow=]
 2229 |   *slash = '/';
      |   ~~~~~~~^~~~~
cgroups/cgfsng.c:2213:8: note: at offset -13 to 
object ‘attach_cgroup’ with size 23 declared here
 2213 |   char 
attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        ^~~~~~~~~~~~~
cgroups/cgfsng.c:2229:10: error: writing 1 byte 
into a region of size 0 [-Werror=stringop-overflow=]
 2229 |   *slash = '/';
      |   ~~~~~~~^~~~~
cgroups/cgfsng.c:2213:8: note: at offset -13 to 
object ‘attach_cgroup’ with size 23 declared here
 2213 |   char 
attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
      |        ^~~~~~~~~~~~~

Link: 
https://launchpadlibrarian.net/494354168/buildlog_ubuntu-groovy-armhf.lxc_1%3A4.0.4-0ubuntu1_BUILDING.txt.gz
Signed-off-by: Christian Brauner <christian.brau...@ubuntu.com>
---
 src/lxc/cgroups/cgfsng.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/lxc/cgroups/cgfsng.c b/src/lxc/cgroups/cgfsng.c
index 85afc63af9..3ab041e1f9 100644
--- a/src/lxc/cgroups/cgfsng.c
+++ b/src/lxc/cgroups/cgfsng.c
@@ -2216,13 +2216,21 @@ static int cgroup_attach_leaf(const struct lxc_conf 
*conf, int unified_fd, pid_t
 
        do {
                bool rm = false;
-               char attach_cgroup[STRLITERALLEN(".lxc-1000/cgroup.procs") + 1];
+               char attach_cgroup[STRLITERALLEN(".lxc-/cgroup.procs") + 
INTTYPE_TO_STRLEN(int) + 1];
                char *slash;
 
                ret = snprintf(attach_cgroup, sizeof(attach_cgroup), 
".lxc-%d/cgroup.procs", idx);
                if (ret < 0 || (size_t)ret >= sizeof(attach_cgroup))
                        return ret_errno(EIO);
 
+               /*
+                * This shouldn't really happen but the compiler might complain
+                * that a short write would cause a buffer overrun. So be on
+                * the safe side.
+                */
+               if (ret < STRLITERALLEN(".lxc-/cgroup.procs"))
+                       return log_error_errno(-EINVAL, EINVAL, "Unexpected 
short write would cause buffer-overrun");
+
                slash = &attach_cgroup[ret] - STRLITERALLEN("/cgroup.procs");
                *slash = '\0';
 

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