Re: [PATCH 1/3] devmem: change vread()/vwrite() prototype toreturn success or error code

[Wu Fengguang]

On Wed, Sep 16, 2009 at 10:14:25AM +0800, KAMEZAWA Hiroyuki wrote:
> On Wed, 16 Sep 2009 09:39:40 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 
> > Silently ignore all vmalloc area holes in vread()/vwrite(),
> > and report success (or error code in future) to the caller.
> >
> > The original intention is to fix a vwrite() related bug, where
> > it could return 0 which cannot be handled correctly by its caller
> > write_kmem(). Then KAMEZAWA recommends to change the prototype
> > to make the semantics clear.
> >
> > CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
> > CC: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > CC: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> > CC: Ingo Molnar <mingo@xxxxxxx>
> > CC: Tejun Heo <tj@xxxxxxxxxx>
> > CC: Nick Piggin <npiggin@xxxxxxx>
> > CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > ---
> >  drivers/char/mem.c      |   24 ++++++++--------
> >  fs/proc/kcore.c         |    5 ++-
> >  include/linux/vmalloc.h |    6 ++--
> >  mm/nommu.c              |    8 ++---
> >  mm/vmalloc.c            |   55 +++++++++++---------------------------
> >  5 files changed, 40 insertions(+), 58 deletions(-)
> >
> > --- linux-mm.orig/mm/vmalloc.c        2009-09-16 08:52:12.000000000 +0800
> > +++ linux-mm/mm/vmalloc.c     2009-09-16 09:24:11.000000000 +0800
> > @@ -1655,7 +1655,6 @@ EXPORT_SYMBOL(vmalloc_32_user);
> >  static int aligned_vread(char *buf, char *addr, unsigned long count)
> >  {
> >       struct page *p;
> > -     int copied = 0;
> >
> >       while (count) {
> >               unsigned long offset, length;
> > @@ -1685,16 +1684,14 @@ static int aligned_vread(char *buf, char
> >
> >               addr += length;
> >               buf += length;
> > -             copied += length;
> >               count -= length;
> >       }
> > -     return copied;
> > +     return 0;
> >  }
> >
> >  static int aligned_vwrite(char *buf, char *addr, unsigned long count)
> >  {
> >       struct page *p;
> > -     int copied = 0;
> >
> >       while (count) {
> >               unsigned long offset, length;
> > @@ -1722,10 +1719,9 @@ static int aligned_vwrite(char *buf, cha
> >               }
> >               addr += length;
> >               buf += length;
> > -             copied += length;
> >               count -= length;
> >       }
> > -     return copied;
> > +     return 0;
> >  }
> >
> >  /**
> > @@ -1734,9 +1730,7 @@ static int aligned_vwrite(char *buf, cha
> >   *   @addr:          vm address.
> >   *   @count:         number of bytes to be read.
> >   *
> > - *   Returns # of bytes which addr and buf should be increased.
> > - *   (same number to @count). Returns 0 if [addr...addr+count) doesn't
> > - *   includes any intersect with alive vmalloc area.
> > + *   Returns 0 on success.
> 
> Seems to return 0 _always_. My version retunrs false if it's out-of-range.
> plz return false if out-of-vmalloc-range.
> Now, use vmalloc area is exported via /proc/vmallocinfo. Then, it's easy
> to know "valid" vmalloc area. But users cannot know memory holes in
> "valid" vmalloc area. So, vread() returns success even if it founds holes.

OK, that's fair enough.

> Another thought:
> Should we purge vread/vwrite and just use copy_to/from_user ?

I like its simplicity. Though I'm afraid I don't know enough to make useful comments.

> To do that, /proc/kcore should be revisited...
> It has to copy pages one by one in aligned manner.

You know it used to not rely on vread() ;)

> Anyway, it's merge window and not good season for this semantic changes of
> a function.

Agreed. I'll do a bug fix only patch, and leave other decisions to you.

> >   *
> >   *   This function checks that addr is a valid vmalloc'ed area, and
> >   *   copy data from that area to a given buffer. If the given memory range
> > @@ -1744,23 +1738,21 @@ static int aligned_vwrite(char *buf, cha
> >   *   proper area of @buf. If there are memory holes, they'll be zero-filled.
> >   *   IOREMAP area is treated as memory hole and no copy is done.
> >   *
> > - *   If [addr...addr+count) doesn't includes any intersects with alive
> > - *   vm_struct area, returns 0.
> >   *   @buf should be kernel's buffer. Because this function uses KM_USER0,
> >   *   the caller should guarantee KM_USER0 is not used.
> >   *
> >   *   Note: In usual ops, vread() is never necessary because the caller
> >   *   should know vmalloc() area is valid and can use memcpy().
> >   *   This is for routines which have to access vmalloc area without
> > - *   any informaion, as /dev/kmem.
> > + *   any information, as /dev/kmem and /dev/kcore.
> >   *
> >   */
> >
> > -long vread(char *buf, char *addr, unsigned long count)
> > +int vread(char *buf, char *addr, unsigned long count)
> >  {
> >       struct vm_struct *tmp;
> > -     char *vaddr, *buf_start = buf;
> > -     unsigned long buflen = count;
> > +     char *vaddr;
> > +     char *buf_end = buf + count;
> >       unsigned long n;
> >
> >       /* Don't allow overflow */
> > @@ -1794,13 +1786,11 @@ long vread(char *buf, char *addr, unsign
> >  finished:
> >       read_unlock(&vmlist_lock);
> >
> > -     if (buf == buf_start)
> > -             return 0;
> >       /* zero-fill memory holes */
> > -     if (buf != buf_start + buflen)
> > -             memset(buf, 0, buflen - (buf - buf_start));
> > +     if (buf != buf_end)
> > +             memset(buf, 0, buf_end - buf);
> >
> > -     return buflen;
> > +     return 0;
> >  }
> >
> >  /**
> > @@ -1809,10 +1799,7 @@ finished:
> >   *   @addr:          vm address.
> >   *   @count:         number of bytes to be read.
> >   *
> > - *   Returns # of bytes which addr and buf should be incresed.
> > - *   (same number to @count).
> > - *   If [addr...addr+count) doesn't includes any intersect with valid
> > - *   vmalloc area, returns 0.
> > + *   Returns 0 on success.
> >   *
> >   *   This function checks that addr is a valid vmalloc'ed area, and
> >   *   copy data from a buffer to the given addr. If specified range of
> > @@ -1820,30 +1807,24 @@ finished:
> >   *   proper area of @buf. If there are memory holes, no copy to hole.
> >   *   IOREMAP area is treated as memory hole and no copy is done.
> >   *
> > - *   If [addr...addr+count) doesn't includes any intersects with alive
> > - *   vm_struct area, returns 0.
> >   *   @buf should be kernel's buffer. Because this function uses KM_USER0,
> >   *   the caller should guarantee KM_USER0 is not used.
> >   *
> >   *   Note: In usual ops, vwrite() is never necessary because the caller
> >   *   should know vmalloc() area is valid and can use memcpy().
> >   *   This is for routines which have to access vmalloc area without
> > - *   any informaion, as /dev/kmem.
> > - *
> > - *   The caller should guarantee KM_USER1 is not used.
> > + *   any information, as /dev/kmem.
> >   */
> >
> > -long vwrite(char *buf, char *addr, unsigned long count)
> > +int vwrite(char *buf, char *addr, unsigned long count)
> >  {
> >       struct vm_struct *tmp;
> >       char *vaddr;
> > -     unsigned long n, buflen;
> > -     int copied = 0;
> > +     unsigned long n;
> >
> >       /* Don't allow overflow */
> >       if ((unsigned long) addr + count < count)
> >               count = -(unsigned long) addr;
> > -     buflen = count;
> >
> >       read_lock(&vmlist_lock);
> >       for (tmp = vmlist; count && tmp; tmp = tmp->next) {
> > @@ -1860,19 +1841,15 @@ long vwrite(char *buf, char *addr, unsig
> >               n = vaddr + tmp->size - PAGE_SIZE - addr;
> >               if (n > count)
> >                       n = count;
> > -             if (!(tmp->flags & VM_IOREMAP)) {
> > +             if (!(tmp->flags & VM_IOREMAP))
> >                       aligned_vwrite(buf, addr, n);
> > -                     copied++;
> > -             }
> >               buf += n;
> >               addr += n;
> >               count -= n;
> >       }
> >  finished:
> >       read_unlock(&vmlist_lock);
> > -     if (!copied)
> > -             return 0;
> > -     return buflen;
> > +     return 0;
> >  }
> >
> >  /**
> > --- linux-mm.orig/include/linux/vmalloc.h     2009-09-16 08:52:12.000000000 +0800
> > +++ linux-mm/include/linux/vmalloc.h  2009-09-16 08:52:17.000000000 +0800
> > @@ -104,9 +104,9 @@ extern void unmap_kernel_range(unsigned
> >  extern struct vm_struct *alloc_vm_area(size_t size);
> >  extern void free_vm_area(struct vm_struct *area);
> >
> > -/* for /dev/kmem */
> > -extern long vread(char *buf, char *addr, unsigned long count);
> > -extern long vwrite(char *buf, char *addr, unsigned long count);
> > +/* for /dev/kmem and /dev/kcore */
> > +extern int vread(char *buf, char *addr, unsigned long count);
> > +extern int vwrite(char *buf, char *addr, unsigned long count);
> >
> >  /*
> >   *   Internals.  Dont't use..
> > --- linux-mm.orig/drivers/char/mem.c  2009-09-16 08:52:12.000000000 +0800
> > +++ linux-mm/drivers/char/mem.c       2009-09-16 09:23:00.000000000 +0800
> > @@ -396,6 +396,7 @@ static ssize_t read_kmem(struct file *fi
> >       unsigned long p = *ppos;
> >       ssize_t low_count, read, sz;
> >       char * kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
> > +     int err = 0;
> >
> >       read = 0;
> >       if (p < (unsigned long) high_memory) {
> > @@ -442,12 +443,12 @@ static ssize_t read_kmem(struct file *fi
> >                       return -ENOMEM;
> >               while (count > 0) {
> >                       sz = size_inside_page(p, count);
> > -                     sz = vread(kbuf, (char *)p, sz);
> > -                     if (!sz)
> > +                     err = vread(kbuf, (char *)p, sz);
> > +                     if (err)
> >                               break;
> >                       if (copy_to_user(buf, kbuf, sz)) {
> > -                             free_page((unsigned long)kbuf);
> > -                             return -EFAULT;
> > +                             err = -EFAULT;
> > +                             break;
> >                       }
> >                       count -= sz;
> >                       buf += sz;
> > @@ -457,7 +458,7 @@ static ssize_t read_kmem(struct file *fi
> >               free_page((unsigned long)kbuf);
> >       }
> >       *ppos = p;
> > -     return read;
> > +     return read ? read : err;
> >  }
> >
> >
> > @@ -521,6 +522,7 @@ static ssize_t write_kmem(struct file *
> >       ssize_t wrote = 0;
> >       ssize_t virtr = 0;
> >       char * kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
> > +     int err = 0;
> >
> >       if (p < (unsigned long) high_memory) {
> >               unsigned long to_write = min_t(unsigned long, count,
> > @@ -543,12 +545,12 @@ static ssize_t write_kmem(struct file *
> >
> >                       n = copy_from_user(kbuf, buf, sz);
> >                       if (n) {
> > -                             if (wrote + virtr)
> > -                                     break;
> > -                             free_page((unsigned long)kbuf);
> > -                             return -EFAULT;
> > +                             err = -EFAULT;
> > +                             break;
> >                       }
> > -                     sz = vwrite(kbuf, (char *)p, sz);
> > +                     err = vwrite(kbuf, (char *)p, sz);
> > +                     if (err)
> > +                             break;
> >                       count -= sz;
> >                       buf += sz;
> >                       virtr += sz;
> > @@ -558,7 +560,7 @@ static ssize_t write_kmem(struct file *
> >       }
> >
> >       *ppos = p;
> > -     return virtr + wrote;
> > +     return virtr + wrote ? : err;
> >  }
> >  #endif
> >
> > --- linux-mm.orig/fs/proc/kcore.c     2009-09-16 08:52:12.000000000 +0800
> > +++ linux-mm/fs/proc/kcore.c  2009-09-16 08:52:17.000000000 +0800
> > @@ -492,11 +492,14 @@ read_kcore(struct file *file, char __use
> >                               return -EFAULT;
> >               } else if (is_vmalloc_or_module_addr((void *)start)) {
> >                       char * elf_buf;
> > +                     int err;
> >
> >                       elf_buf = kzalloc(tsz, GFP_KERNEL);
> >                       if (!elf_buf)
> >                               return -ENOMEM;
> > -                     vread(elf_buf, (char *)start, tsz);
> > +                     err = vread(elf_buf, (char *)start, tsz);
> > +                     if (err)
> > +                             return err;
> 
> This is wrong.
> 
> /proc/kcore provides "coredump" image of kernel and it never faults.
> Then, it doesn't check error code intentionally and just return
> zero-filled buffer of reuqested length.

Ah OK. Will revert this change.

Thanks,
Fengguang

> >                       /* we have to zero-fill user buffer even if no read */
> >                       if (copy_to_user(buffer, elf_buf, tsz)) {
> >                               kfree(elf_buf);
> > --- linux-mm.orig/mm/nommu.c  2009-09-16 09:01:36.000000000 +0800
> > +++ linux-mm/mm/nommu.c       2009-09-16 09:02:02.000000000 +0800
> > @@ -263,20 +263,20 @@ unsigned long vmalloc_to_pfn(const void
> >  }
> >  EXPORT_SYMBOL(vmalloc_to_pfn);
> >
> > -long vread(char *buf, char *addr, unsigned long count)
> > +int vread(char *buf, char *addr, unsigned long count)
> >  {
> >       memcpy(buf, addr, count);
> > -     return count;
> > +     return 0;
> >  }
> >
> > -long vwrite(char *buf, char *addr, unsigned long count)
> > +int vwrite(char *buf, char *addr, unsigned long count)
> >  {
> >       /* Don't allow overflow */
> >       if ((unsigned long) addr + count < count)
> >               count = -(unsigned long) addr;
> >
> >       memcpy(addr, buf, count);
> > -     return(count);
> > +     return 0;
> >  }
> >
> >  /*
> >
> > --
> >
> >
--
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