Re: TMPFS over NFSv4

[Hugh Dickins]

Hi Greg,

On Mon, 24 May 2010, Alan Cox wrote:
> On Mon, 24 May 2010 02:57:30 -0700
> Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > On Mon, May 24, 2010 at 2:26 AM, Tharindu Rukshan Bamunuarachchi
> > <btharindu@xxxxxxxxx> wrote:
> > > thankx a lot Hugh ... I will try this out ... (bit harder patch
> > > already patched SLES kernel :-p ) ....
> > 
> > If patch conflicts are a problem, you really only need to put in the
> > two-liner patch to mm/mmap.c: Alan was seeking perfection in
> > the rest of the patch, but you can get away without it.
> > 
> > >
> > > BTW, what does Alan means by "strict overcommit" ?
> > 
> > Ah, that phrase, yes, it's a nonsense, but many of us do say it by mistake.
> > Alan meant to say "strict no-overcommit".
> 
> No I always meant to say 'strict overcommit'. It avoids excess negatives
> and "no noovercommit" discussions.
> 
> I guess 'strict overcommit control' would have been clearer 8)
> 
> Alan

I see we've just missed 2.6.27.47-rc1, but if there's to be an -rc2,
please include Alan's 2.6.28 oops fix below: which Tharindu appears
to be needing - just now discussed on linux-mm and linux-nfs.
Failing that, please queue it up for 2.6.27.48.

Or if you'd prefer a smaller patch for -stable, then just the mm/mmap.c
part of it should suffice: I think it's fair to say that the rest of the
patch was more precautionary - as Alan describes, for catching other bugs,
so good for an ongoing development tree, but not necessarily in -stable.
(However, Alan may disagree - I've already misrepresented him once here!)

Thanks,
Hugh

commit 731572d39fcd3498702eda4600db4c43d51e0b26
Author: Alan Cox <alan@xxxxxxxxxx>
Date:   Wed Oct 29 14:01:20 2008 -0700

    nfsd: fix vm overcommit crash
    
    Junjiro R.  Okajima reported a problem where knfsd crashes if you are
    using it to export shmemfs objects and run strict overcommit.  In this
    situation the current->mm based modifier to the overcommit goes through a
    NULL pointer.
    
    We could simply check for NULL and skip the modifier but we've caught
    other real bugs in the past from mm being NULL here - cases where we did
    need a valid mm set up (eg the exec bug about a year ago).
    
    To preserve the checks and get the logic we want shuffle the checking
    around and add a new helper to the vm_ security wrappers
    
    Also fix a current->mm reference in nommu that should use the passed mm
    
    [akpm@xxxxxxxxxxxxxxxxxxxx: coding-style fixes]
    [akpm@xxxxxxxxxxxxxxxxxxxx: fix build]
    Reported-by: Junjiro R. Okajima <hooanon05@xxxxxxxxxxx>
    Acked-by: James Morris <jmorris@xxxxxxxxx>
    Signed-off-by: Alan Cox <alan@xxxxxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

diff --git a/include/linux/security.h b/include/linux/security.h
index f5c4a51..c13f1ce 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1585,6 +1585,7 @@ int security_syslog(int type);
 int security_settime(struct timespec *ts, struct timezone *tz);
 int security_vm_enough_memory(long pages);
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
+int security_vm_enough_memory_kern(long pages);
 int security_bprm_alloc(struct linux_binprm *bprm);
 void security_bprm_free(struct linux_binprm *bprm);
 void security_bprm_apply_creds(struct linux_binprm *bprm, int unsafe);
@@ -1820,6 +1821,11 @@ static inline int security_vm_enough_memory(long pages)
 	return cap_vm_enough_memory(current->mm, pages);
 }
 
+static inline int security_vm_enough_memory_kern(long pages)
+{
+	return cap_vm_enough_memory(current->mm, pages);
+}
+
 static inline int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
 {
 	return cap_vm_enough_memory(mm, pages);
diff --git a/mm/mmap.c b/mm/mmap.c
index 74f4d15..de14ac2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -175,7 +175,8 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 
 	/* Don't let a single process grow too big:
 	   leave 3% of the size of this process for other processes */
-	allowed -= mm->total_vm / 32;
+	if (mm)
+		allowed -= mm->total_vm / 32;
 
 	/*
 	 * cast `allowed' as a signed long because vm_committed_space
diff --git a/mm/nommu.c b/mm/nommu.c
index 2696b24..7695dc8 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1454,7 +1454,8 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 
 	/* Don't let a single process grow too big:
 	   leave 3% of the size of this process for other processes */
-	allowed -= current->mm->total_vm / 32;
+	if (mm)
+		allowed -= mm->total_vm / 32;
 
 	/*
 	 * cast `allowed' as a signed long because vm_committed_space
diff --git a/mm/shmem.c b/mm/shmem.c
index d38d7e6..0ed0752 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -161,8 +161,8 @@ static inline struct shmem_sb_info *SHMEM_SB(struct super_block *sb)
  */
 static inline int shmem_acct_size(unsigned long flags, loff_t size)
 {
-	return (flags & VM_ACCOUNT)?
-		security_vm_enough_memory(VM_ACCT(size)): 0;
+	return (flags & VM_ACCOUNT) ?
+		security_vm_enough_memory_kern(VM_ACCT(size)) : 0;
 }
 
 static inline void shmem_unacct_size(unsigned long flags, loff_t size)
@@ -179,8 +179,8 @@ static inline void shmem_unacct_size(unsigned long flags, loff_t size)
  */
 static inline int shmem_acct_block(unsigned long flags)
 {
-	return (flags & VM_ACCOUNT)?
-		0: security_vm_enough_memory(VM_ACCT(PAGE_CACHE_SIZE));
+	return (flags & VM_ACCOUNT) ?
+		0 : security_vm_enough_memory_kern(VM_ACCT(PAGE_CACHE_SIZE));
 }
 
 static inline void shmem_unacct_blocks(unsigned long flags, long pages)
diff --git a/security/security.c b/security/security.c
index 255b085..c0acfa7 100644
--- a/security/security.c
+++ b/security/security.c
@@ -198,14 +198,23 @@ int security_settime(struct timespec *ts, struct timezone *tz)
 
 int security_vm_enough_memory(long pages)
 {
+	WARN_ON(current->mm == NULL);
 	return security_ops->vm_enough_memory(current->mm, pages);
 }
 
 int security_vm_enough_memory_mm(struct mm_struct *mm, long pages)
 {
+	WARN_ON(mm == NULL);
 	return security_ops->vm_enough_memory(mm, pages);
 }
 
+int security_vm_enough_memory_kern(long pages)
+{
+	/* If current->mm is a kernel thread then we will pass NULL,
+	   for this specific case that is fine */
+	return security_ops->vm_enough_memory(current->mm, pages);
+}
+
 int security_bprm_alloc(struct linux_binprm *bprm)
 {
 	return security_ops->bprm_alloc_security(bprm);
--
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