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SubjectRe: [PATCH] devmem: handle partial kmem write/read
On Tue, Sep 15, 2009 at 10:31:13AM +0800, KAMEZAWA Hiroyuki wrote:
> On Tue, 15 Sep 2009 10:02:08 +0800
> Wu Fengguang <fengguang.wu@intel.com> wrote:
>
> > On Tue, Sep 15, 2009 at 08:24:48AM +0800, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 14 Sep 2009 12:34:44 +0800
> > > Wu Fengguang <fengguang.wu@intel.com> wrote:
> > >
> > > > Hi Kame,
> > > >
> > > > This patch needs more work. I first intent to fix a bug:
> > > >
> > > > sz = vwrite(kbuf, (char *)p, sz);
> > > > p += sz;
> > > > }
> > > >
> > > > So if the returned len is 0, the kbuf/p pointers will mismatch.
> > > >
> > > > Then I realize it changed the write behavior. The current vwrite()
> > > > behavior is strange, it returns 0 if _whole range_ is hole, otherwise
> > > > ignores the hole silently. So holes can be treated differently even in
> > > > the original code.
> > > >
> > > Ah, ok...
> > >
> > > > I'm not really sure about the right behavior. KAME-san, do you have
> > > > any suggestions?
> > > >
> > > maybe following make sense.
> > > =
> > > written = vwrite(kbuf, (char *p), sz);
> > > if (!written) // whole vmem was a hole
> > > written = sz;
> >
> > Since the 0 return value won't be used at all, it would be simpler to
> > tell vwrite() return the untouched buflen/sz, like this. It will ignore
> > _all_ the holes silently. Need to update comments too.
> >
>
> Hmm. IIUC the original kmem code returns immediately if vread/vwrite returns 0.

Agreed for vread. It seems vwrite don't do that for both kmem and
vmalloc part.

> But it seems this depends on wrong assumption that vmap area is continuous,
> at the first look.

You mean it is possible that not all pages are mapped in a vmlist addr,size range?
Is vmalloc areas guaranteed to be page aligned? Sorry for the dumb questions.

> In man(4) mem,kmem
> ==
> Byte addresses in mem are interpreted as physical memory addresses.
> References to nonexistent locations cause errors to be returned.
> .....
> The file kmem is the same as mem, except that the kernel virtual memory
> rather than physical memory is accessed.
>
> ==
>
> Then, we have to return error for accesses to "nonexistent locations".

That's one important factor to consider. On the other hand, the
original kmem read/write implementation don't return error code for
holes. Instead kmem read returns early, while kmem write ignores holes
but is buggy..

> memory-hole in vmap area is ....."nonexistent" ?
> I think it's nonexistent if there are no overlaps between requested [pos, pos+len)
> and registerred vmalloc area.
> But, hmm, there are no way for users to know "existing vmalloc area".
> Then, my above idea may be wrong.
>
> Then, I'd like to modify as following,
>
> - If is_vmalloc_or_module_addr(requested address) is false, return -EFAULT.
> - If is_vmalloc_or_module_addr(requested address) is true, return no error.
> Even if specified range doesn't include no exsiting vmalloc area.
>
> How do you think ?

Looks reasonable to me. But it's good to hear more wide opinions..

> Thanks,
> -Kame
> p.s. I wonder current /dev/kmem cannot read text area of kernel if it's not
> directly mapped.

Attached is a small tool (from LKML) for reading 'modprobe_path' from kmem,
it's not text, but is close..

Thanks,
Fengguang

>
>
> > --- linux-mm.orig/mm/vmalloc.c 2009-09-15 09:40:08.000000000 +0800
> > +++ linux-mm/mm/vmalloc.c 2009-09-15 09:43:33.000000000 +0800
> > @@ -1834,7 +1834,6 @@ long vwrite(char *buf, char *addr, unsig
> > struct vm_struct *tmp;
> > char *vaddr;
> > unsigned long n, buflen;
> > - int copied = 0;
> >
> > /* Don't allow overflow */
> > if ((unsigned long) addr + count < count)
> > @@ -1858,7 +1857,6 @@ long vwrite(char *buf, char *addr, unsig
> > n = count;
> > if (!(tmp->flags & VM_IOREMAP)) {
> > aligned_vwrite(buf, addr, n);
> > - copied++;
> > }
> > buf += n;
> > addr += n;
> > @@ -1866,8 +1864,6 @@ long vwrite(char *buf, char *addr, unsig
> > }
> > finished:
> > read_unlock(&vmlist_lock);
> > - if (!copied)
> > - return 0;
> > return buflen;
> > }
> >
> > > ==
> > > needs fix.
> > >
> > > Anyway, I wonder kmem is broken. It's should be totally rewritten.
> > >
> > > For example, this doesn't check anything.
> > > ==
> > > if (p < (unsigned long) high_memory) {
> > >
> > > ==
> > >
> > > But....are there users ?
> > > If necessary, I'll write some...
> >
> > I'm trying to stop possible mem/kmem users to access hwpoison pages..
> > I'm not the user, but rather a tester ;)
> >
> > Thanks,
> > Fengguang
> >
> > > Thanks,
> > > -Kame
> > >
> > >
> > > > Thanks,
> > > > Fengguang
> > > >
> > > > On Mon, Sep 14, 2009 at 11:29:01AM +0800, Wu Fengguang wrote:
> > > > > Current vwrite silently ignores vwrite() to vmap holes.
> > > > > Better behavior should be stop the write and return
> > > > > on such holes.
> > > > >
> > > > > Also return on partial read, which may happen in future
> > > > > (eg. hit hwpoison pages).
> > > > >
> > > > > CC: Andi Kleen <andi@firstfloor.org>
> > > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > > > ---
> > > > > drivers/char/mem.c | 30 ++++++++++++++++++------------
> > > > > 1 file changed, 18 insertions(+), 12 deletions(-)
> > > > >
> > > > > --- linux-mm.orig/drivers/char/mem.c 2009-09-14 10:59:50.000000000 +0800
> > > > > +++ linux-mm/drivers/char/mem.c 2009-09-14 11:06:25.000000000 +0800
> > > > > @@ -444,18 +444,22 @@ static ssize_t read_kmem(struct file *fi
> > > > > if (!kbuf)
> > > > > return -ENOMEM;
> > > > > while (count > 0) {
> > > > > + unsigned long n;
> > > > > +
> > > > > sz = size_inside_page(p, count);
> > > > > - sz = vread(kbuf, (char *)p, sz);
> > > > > - if (!sz)
> > > > > + n = vread(kbuf, (char *)p, sz);
> > > > > + if (!n)
> > > > > break;
> > > > > - if (copy_to_user(buf, kbuf, sz)) {
> > > > > + if (copy_to_user(buf, kbuf, n)) {
> > > > > free_page((unsigned long)kbuf);
> > > > > return -EFAULT;
> > > > > }
> > > > > - count -= sz;
> > > > > - buf += sz;
> > > > > - read += sz;
> > > > > - p += sz;
> > > > > + count -= n;
> > > > > + buf += n;
> > > > > + read += n;
> > > > > + p += n;
> > > > > + if (n < sz)
> > > > > + break;
> > > > > }
> > > > > free_page((unsigned long)kbuf);
> > > > > }
> > > > > @@ -551,11 +555,13 @@ static ssize_t write_kmem(struct file *
> > > > > free_page((unsigned long)kbuf);
> > > > > return -EFAULT;
> > > > > }
> > > > > - sz = vwrite(kbuf, (char *)p, sz);
> > > > > - count -= sz;
> > > > > - buf += sz;
> > > > > - virtr += sz;
> > > > > - p += sz;
> > > > > + n = vwrite(kbuf, (char *)p, sz);
> > > > > + count -= n;
> > > > > + buf += n;
> > > > > + virtr += n;
> > > > > + p += n;
> > > > > + if (n < sz)
> > > > > + break;
> > > > > }
> > > > > free_page((unsigned long)kbuf);
> > > > > }
> > > > --
> > > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > > > the body of a message to majordomo@vger.kernel.org
> > > > More majordomo info at http://vger.kernel.org/majordomo-info.html
> > > > Please read the FAQ at http://www.tux.org/lkml/
> > > >
> >
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/poll.h>
#include <sys/mman.h>

int page_size;
#define PAGE_SIZE page_size
#define PAGE_MASK (~(PAGE_SIZE-1))

void get_var (unsigned long addr) {
off_t ptr = addr & ~(PAGE_MASK);
off_t offset = addr & PAGE_MASK;
int i = 0;
char *map;
static int kfd = -1;

kfd = open("/dev/kmem",O_RDONLY);
if (kfd < 0) {
perror("open");
exit(0);
}

map = mmap(NULL,PAGE_SIZE,PROT_READ,MAP_SHARED,kfd,offset);
if (map == MAP_FAILED) {
perror("mmap");
exit(-1);
}
printf("%s\n",map+ptr);

return;
}

int main(int argc, char **argv)
{
FILE *fp;
char addr_str[11]="0x";
char var[51];
unsigned long addr;
char ch;
int r;

if (argc != 2) {
fprintf(stderr,"usage: %s System.map\n",argv[0]);
exit(-1);
}

if ((fp = fopen(argv[1],"r")) == NULL) {
perror("fopen");
exit(-1);
}

do {
/* ffffffff81723880 D modprobe_path */
r = fscanf(fp,"%16s %c %50s\n",&addr_str[2],&ch,var);
if (strcmp(var,"modprobe_path")==0)
break;
} while(r > 0);
if (r < 0) {
printf("could not find modprobe_path\n");
exit(-1);
}
page_size = getpagesize();
addr = strtoul(addr_str,NULL,16);
printf("found modprobe_path at (%s) %08lx\n",addr_str,addr);
get_var(addr);

return 0;
}

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