Re: [PATCH next] fs/hfsplus/xattr: Use memcpy() and strscpy() to build xattr_name

[David Laight <david.laight.linux@gmail.com>]

On Mon, 15 Jun 2026 23:33:09 -0700
Viacheslav Dubeyko <slava@dubeyko.com> wrote:

> On Wed, 2026-06-10 at 21:18 -0700, Darrick J. Wong wrote:
> > On Wed, Jun 10, 2026 at 08:50:33PM -0700, Viacheslav Dubeyko wrote:  
> > > On Mon, 2026-06-08 at 10:55 +0100,
> > > david.laight.linux@gmail.com wrote:  
> > > > From: David Laight <david.laight.linux@gmail.com>
> > > > 
> > > > xattr_name is kmalloc()ed at the (assumed) maximal size and then
> > > > the
> > > > prefix
> > > > and name concatenated together.
> > > > Use memcpy() for the prefix - its length is passed and strscpy()
> > > > for
> > > > the
> > > > name to ensure it really doesnt overflow.
> > > > 
> > > > Prior to bf29e886b242c the buffers were smaller and on-stack.
> > > > (But I cant see the copy in the old code.)
> > > > I am also not sure why the buffer isnt created "just long
> > > > enough".
> > > > 
> > > > Signed-off-by: David Laight <david.laight.linux@gmail.com>
> > > > ---
> > > > This is one of a group of patches that remove potentially
> > > > unbounded
> > > > strcpy() calls.
> > > > 
> > > > They are mostly replaced by strscpy() or, when strlen() has just
> > > > been
> > > > called, with memcpy() (usually including the '\0').
> > > > 
> > > > Calls with copy string literals into arrays are left unchanged.
> > > > They are safe and easily detected as such.
> > > > 
> > > > The changes were made by getting the compiler to detect the calls
> > > > and
> > > > then fixing the code by hand.
> > > > 
> > > > Note that all the changes are only compile tested.
> > > > 
> > > > Some Makefiles were changed to allow files to contain strcpy().
> > > > As well as 'difficult to fix' files, this included 'show'
> > > > functions
> > > > as they really need to use sysfs_emit() or seq_printf().
> > > > 
> > > > All the patches are being sent individually to avoid very long cc
> > > > lists.
> > > > Apologies for the terse commit messages and likely unexpected
> > > > tags.
> > > > (There are about 100 patches in total.)
> > > > 
> > > >  fs/hfsplus/xattr.c | 12 ++++++------
> > > >  1 file changed, 6 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/fs/hfsplus/xattr.c b/fs/hfsplus/xattr.c
> > > > index 452a1f9becb2..0b3dd48c28c9 100644
> > > > --- a/fs/hfsplus/xattr.c
> > > > +++ b/fs/hfsplus/xattr.c
> > > > @@ -550,8 +550,8 @@ int hfsplus_setxattr(struct inode *inode,
> > > > const
> > > > char *name,
> > > >  	xattr_name = kmalloc(xattr_name_len, GFP_KERNEL);
> > > >  	if (!xattr_name)
> > > >  		return -ENOMEM;
> > > > -	strcpy(xattr_name, prefix);
> > > > -	strcpy(xattr_name + prefixlen, name);
> > > > +	memcpy(xattr_name, prefix, prefixlen);  
> > > 
> > > What's the point to mix memcpy and str*() family of methods? What's
> > > wrong with str*() method here? Otherwise, if it is wrong to use
> > > str*()
> > > family of methods, then why is it correct to use for second
> > > operation?
> > >   
> > > > +	strscpy(xattr_name + prefixlen, name, xattr_name_len -
> > > > prefixlen);  
> > > 
> > > Why strscpy() is better than strncpy()? What is the main argument
> > > here?
> > >   
> > > >  	res = __hfsplus_setxattr(inode, xattr_name, value, size,
> > > > flags);
> > > >  	kfree(xattr_name);
> > > >  
> > > > @@ -698,6 +698,7 @@ ssize_t hfsplus_getxattr(struct inode *inode,
> > > > const char *name,
> > > >  			 void *value, size_t size,
> > > >  			 const char *prefix, size_t prefixlen)
> > > >  {
> > > > +	size_t xattr_name_len = NLS_MAX_CHARSET_SIZE *
> > > > HFSPLUS_ATTR_MAX_STRLEN + 1;  
> > > 
> > > Frankly speaking, it looks like a constant that should be declared
> > > in
> > > hfs_common.h. Even if we would like to declare it here, then it
> > > should
> > > be const size_t, from my point of view.
> > >   
> > > >  	int res;
> > > >  	char *xattr_name;
> > > >  
> > > > @@ -705,13 +706,12 @@ ssize_t hfsplus_getxattr(struct inode
> > > > *inode,
> > > > const char *name,
> > > >  		inode->i_ino, name ? name : NULL,
> > > >  		prefix ? prefix : NULL);
> > > >  
> > > > -	xattr_name = kmalloc(NLS_MAX_CHARSET_SIZE *
> > > > HFSPLUS_ATTR_MAX_STRLEN + 1,
> > > > -			     GFP_KERNEL);
> > > > +	xattr_name = kmalloc(xattr_name_len, GFP_KERNEL);  
> > > 
> > > Finally, I think kzalloc() should be much better for both cases.  
> > 
> > kasprintf()?  
> > >   
> 
> It sounds much better than suggested fix.

If performance matters here it will be a lot slower.
The snprintf() code itself is slow and kasprintf() has to do it twice.
(As well as looking at the strings twice.)

It also only allocates a buffer that is big enough for a single
terminating '\0' - and (at least some versions) of this code zero
the rest of the buffer (possibly to avoid a bug).

One option would be something like kstrdup() that concatenates two
strings.

	David

> 
> Thanks,
> Slava.