Re: [PATCH 1/3] ipc/sem.c: remove sem_base, embed struct sem

[Andrew Morton]

On Mon, 15 May 2017 19:19:10 +0200 Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:

> sma->sem_base is initialized with
> 	sma->sem_base = (struct sem *) &sma[1];
> 
> The current code has four problems:
> - There is an unnecessary pointer dereference - sem_base is not needed.
> - Alignment for struct sem only works by chance.
> - The current code causes false positive for static code analysis.
> - This is a cast between different non-void types, which the future
>   randstruct GCC plugin warns on.
> 
> And, as bonus, the code size gets smaller:
> 
> Before:
>   0 .text         00003770
> After:
>   0 .text         0000374e


This clashes with Kees's patch, below.  Does it have the same effect?


From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: ipc/sem: avoid indexing past end of sem_array

This changes the struct + trailing data pattern to using a void * so that
the end of sem_array is found without possibly indexing past the end which
can upset some static analyzers.  Mostly, this ends up avoiding a cast
between different non-void types, which the future randstruct GCC plugin
was warning about.

Link: 20170508222345.GA52073@beast">http://lkml.kernel.org/r/20170508222345.GA52073@beast
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 ipc/sem.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff -puN ipc/sem.c~ipc-sem-avoid-indexing-past-end-of-sem_array ipc/sem.c
--- a/ipc/sem.c~ipc-sem-avoid-indexing-past-end-of-sem_array
+++ a/ipc/sem.c
@@ -475,6 +475,7 @@ static int newary(struct ipc_namespace *
 {
 	int id;
 	int retval;
+	void *sem_alloc;
 	struct sem_array *sma;
 	int size;
 	key_t key = params->key;
@@ -488,11 +489,14 @@ static int newary(struct ipc_namespace *
 		return -ENOSPC;
 
 	size = sizeof(*sma) + nsems * sizeof(struct sem);
-	sma = ipc_rcu_alloc(size);
-	if (!sma)
+	sem_alloc = ipc_rcu_alloc(size);
+	if (!sem_alloc)
 		return -ENOMEM;
 
-	memset(sma, 0, size);
+	memset(sem_alloc, 0, size);
+
+	sma = sem_alloc;
+	sma->sem_base = sem_alloc + sizeof(*sma);
 
 	sma->sem_perm.mode = (semflg & S_IRWXUGO);
 	sma->sem_perm.key = key;
@@ -504,8 +508,6 @@ static int newary(struct ipc_namespace *
 		return retval;
 	}
 
-	sma->sem_base = (struct sem *) &sma[1];
-
 	for (i = 0; i < nsems; i++) {
 		INIT_LIST_HEAD(&sma->sem_base[i].pending_alter);
 		INIT_LIST_HEAD(&sma->sem_base[i].pending_const);
_