[Linux-ia64] /proc/pid/mem and stack variables

[Linux-ia64] /proc/pid/mem and stack variables

[Maciej Golebiewski]

Hello everyone,

I'm new on this list. I looked through the archives but could not
find answer to my problem.

I have a library that makes use of /proc/pid/mem entries in the
/proc pseudo-filesystem. It runs perfectly on IA32 under 2.2.12
(and some superficial tests suggest that it works under
2.4.0-test12 as well).

I have recompiled the library on IA64 under 2.4.0-test12
and using version 2.9-ia64-000216-final. The library works
OK except when the address it tries to access via /proc/pid/mem
refers to variables allocated on stack (e.g. local arrays defined
in functions).

This is a snippet of code used to access memory via /proc/pid/mem:

void mem_read (int fd, void *buf, void *addr, size_t len) {

	if (lseek (fd, (off_t)addr, SEEK_SET) == (off_t)-1)
                perror ("lseek");
	read (fd, buf, len);

lseek in this code fails (EINVAL) always when addr is address of
a variable allocated on stack.

I have no idea if the problem is because I'm doing something
wrong, or because /proc/pid/mem is not yet fully implemented on IA64.
I appreciate any hints and help with this.

Thanks in advance,

Maciej

-- 
Maciej Go³êbiewski
golebiewski@ccrl-nece.de
http://www.ccrl-nece.technopark.gmd.de/~maciej
C&C Research Laboratories, NEC Europe Ltd., Sankt Augustin, Germany

Re: [Linux-ia64] /proc/pid/mem and stack variables

[Pete Wyckoff]

maciej@ccrl-nece.technopark.gmd.de said:
> I have recompiled the library on IA64 under 2.4.0-test12
> and using version 2.9-ia64-000216-final. The library works
> OK except when the address it tries to access via /proc/pid/mem
> refers to variables allocated on stack (e.g. local arrays defined
> in functions).
> 
> This is a snippet of code used to access memory via /proc/pid/mem:
> 
> void mem_read (int fd, void *buf, void *addr, size_t len) {
> 
> 	if (lseek (fd, (off_t)addr, SEEK_SET) = (off_t)-1)
>                 perror ("lseek");
> 	read (fd, buf, len);
> 
> lseek in this code fails (EINVAL) always when addr is address of
> a variable allocated on stack.

Try removing the "offset >= 0" check in default_llseek in
fs/read_write.c.  Stack pages seem to have the high bit set which
is flummoxing that test.

If you keep this "solution" you may want to write an llseek function
for /proc/pid/mem so that the entire kernel doesn't suffer any unforseen
consequences.

		-- Pete

Re: [Linux-ia64] /proc/pid/mem and stack variables

[David Mosberger]

>>>>> On Fri, 12 Jan 2001 13:45:10 -0500, Pete Wyckoff <pw@osc.edu> said:

  Pete> maciej@ccrl-nece.technopark.gmd.de said:
  >> I have recompiled the library on IA64 under 2.4.0-test12 and
  >> using version 2.9-ia64-000216-final. The library works OK except
  >> when the address it tries to access via /proc/pid/mem refers to
  >> variables allocated on stack (e.g. local arrays defined in
  >> functions).
  >> 
  >> This is a snippet of code used to access memory via
  >> /proc/pid/mem:
  >> 
  >> void mem_read (int fd, void *buf, void *addr, size_t len) {
  >> 
  >> if (lseek (fd, (off_t)addr, SEEK_SET) = (off_t)-1) perror
  >> ("lseek"); read (fd, buf, len);
  >> 
  >> lseek in this code fails (EINVAL) always when addr is address of
  >> a variable allocated on stack.

  Pete> Try removing the "offset >= 0" check in default_llseek in
  Pete> fs/read_write.c.  Stack pages seem to have the high bit set
  Pete> which is flummoxing that test.

  Pete> If you keep this "solution" you may want to write an llseek
  Pete> function for /proc/pid/mem so that the entire kernel doesn't
  Pete> suffer any unforseen consequences.

Yes, implementing llseek for fs/proc/base.c:proc_mem_operations is the
right solution.  /dev/kmem already has its own llseek (which treats
the offset as unsigned) but, for some reason, was left out of the
/proc/pid/mem support.

Don, can you add this to the TODO list so we won't forget about it?

Thanks,

	--david

Re: [Linux-ia64] /proc/pid/mem and stack variables

[Maciej Golebiewski]

On Fri, Jan 12, 2001 at 06:29:33PM -0800, David Mosberger wrote:
> >>>>> On Fri, 12 Jan 2001 13:45:10 -0500, Pete Wyckoff <pw@osc.edu> said:
> Yes, implementing llseek for fs/proc/base.c:proc_mem_operations is the
> right solution.  /dev/kmem already has its own llseek (which treats
> the offset as unsigned) but, for some reason, was left out of the
> /proc/pid/mem support.
> 
> Don, can you add this to the TODO list so we won't forget about it?

Thanks a lot for help. It seems that the solution is simpler than I
was expecting. Anyway, I don't have a root access to the IA64 machine
in question, so I can't test it myself, but one of the guys with root
access told me he'll try to find some time and try it out.

Once again thank a lot.

Maciej

-- 
Maciej Go³êbiewski
golebiewski@ccrl-nece.de
http://www.ccrl-nece.technopark.gmd.de/~maciej
C&C Research Laboratories, NEC Europe Ltd., Sankt Augustin, Germany

Re: [Linux-ia64] /proc/pid/mem and stack variables

[Erich Focht]

A small patch for the file fs/proc/base.c implementing llseek for
/proc/PID/mem is appended. It's the same as for /dev/kmem therefore it
does only SEEK_SET and SEEK_CUR, not SEEK_END. 

It works for reading from stack pages, positioning is ok but the return
value is -1 (which is wrong). Probably because the offsets seem to be
negative... Can anybody please tell me why I'm getting the -1 error return
though I should get back the huge negative offset?

Thanks in advance,
Erich


On Mon, 15 Jan 2001, Maciej Golebiewski wrote:

> On Fri, Jan 12, 2001 at 06:29:33PM -0800, David Mosberger wrote:
> > >>>>> On Fri, 12 Jan 2001 13:45:10 -0500, Pete Wyckoff <pw@osc.edu> said:
> > Yes, implementing llseek for fs/proc/base.c:proc_mem_operations is the
> > right solution.  /dev/kmem already has its own llseek (which treats
> > the offset as unsigned) but, for some reason, was left out of the
> > /proc/pid/mem support.
> > 
> > Don, can you add this to the TODO list so we won't forget about it?
> 
> Thanks a lot for help. It seems that the solution is simpler than I
> was expecting. Anyway, I don't have a root access to the IA64 machine
> in question, so I can't test it myself, but one of the guys with root
> access told me he'll try to find some time and try it out.



*** linux-2.4.0test12/fs/proc/base.c.orig1      Mon Jan 15 13:12:36 2001
--- linux-2.4.0test12/fs/proc/base.c    Mon Jan 15 13:14:49 2001
***************
*** 396,402 ****
--- 396,417 ----
  }
  #endif
  
+ static loff_t mem_lseek(struct file * file, loff_t offset, int orig)
+ {
+         switch (orig) {
+                 case 0:
+                         file->f_pos = offset;
+                         return file->f_pos;
+                 case 1:
+                         file->f_pos += offset;
+                         return file->f_pos;
+                 default:
+                         return -EINVAL;
+         }
+ }
+ 
  static struct file_operations proc_mem_operations = {
+       llseek:         mem_lseek,
        read:           mem_read,
        write:          mem_write,
  };

Re: [Linux-ia64] /proc/pid/mem and stack variables

[David Mosberger]

>>>>> On Mon, 15 Jan 2001 18:57:56 +0100 (MET), Erich Focht <focht@ess.nec.de> said:

  Erich> A small patch for the file fs/proc/base.c implementing llseek for
  Erich> /proc/PID/mem is appended. It's the same as for /dev/kmem therefore it
  Erich> does only SEEK_SET and SEEK_CUR, not SEEK_END. 

Thanks for the patch, I applied it to my tree and will work on getting
it into Linus's tree.

  Erich> It works for reading from stack pages, positioning is ok but the return
  Erich> value is -1 (which is wrong). Probably because the offsets seem to be
  Erich> negative... Can anybody please tell me why I'm getting the -1 error return
  Erich> though I should get back the huge negative offset?

I added a force_successful_syscall_return() to ensure that a succesful
lseek() returning a "negative" value isn't mistaken as an error.  I
did the same to memory_lseek() in drivers/char/mem.c.  This should
avoid the problem you were seeing.

	--david