User applications

User applications

[Carsten Langgaard]

I have a few questions about user applications.

When a new user process is started will its user space be cleared by the
kernel or is there a potential leak from an older user process ?
What about the registers values, are they cleared for each new user
application or will it simply contain the current value it got when the
user application is started ?
How can you flush the data and instruction cashes from a user
application ?

/Carsten

--
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Re: User applications

[Michael Shmulevich]

Carsten Langgaard wrote:

> I have a few questions about user applications.
>
> When a new user process is started will its user space be cleared by the
> kernel or is there a potential leak from an older user process ?

Usually it is defied by the loader. If the data section contents is set to
LOAD, then the contents of the section will be loaded from disk (no leak),
if not -- whatever values left i nmemory will be there, or exactly, the
virtual page of some other proccess that was swapped out or ended.

> What about the registers values, are they cleared for each new user
> application or will it simply contain the current value it got when the
> user application is started ?

It depends on the context switch algorithm of the processor, I think.

> How can you flush the data and instruction cashes from a user
> application ?
>

As far as I understand, ASID must take care of it. It contains unique IDs
per process virtual space, so that even
though virtual addresses may be found in TLB, their ASID will be different,
causing TLB miss and probably page fault.

>
> /Carsten
>
> --
> _    _ ____  ___   Carsten Langgaard   Mailto:carstenl@mips.com
> |\  /|||___)(___   MIPS Denmark        Direct: +45 4486 5527
> | \/ |||    ____)  Lautrupvang 4B      Switch: +45 4486 5555
>   TECHNOLOGIES     2750 Ballerup       Fax...: +45 4486 5556
>                    Denmark             http://www.mips.com

Michael.

Re: User applications

[Carsten Langgaard]

Michael Shmulevich wrote:

> Carsten Langgaard wrote:
>
> > I have a few questions about user applications.
> >
> > When a new user process is started will its user space be cleared by the
> > kernel or is there a potential leak from an older user process ?
>
> Usually it is defied by the loader. If the data section contents is set to
> LOAD, then the contents of the section will be loaded from disk (no leak),
> if not -- whatever values left i nmemory will be there, or exactly, the
> virtual page of some other proccess that was swapped out or ended.
>
> > What about the registers values, are they cleared for each new user
> > application or will it simply contain the current value it got when the
> > user application is started ?
>
> It depends on the context switch algorithm of the processor, I think.
>
> > How can you flush the data and instruction cashes from a user
> > application ?
> >
>
> As far as I understand, ASID must take care of it. It contains unique IDs
> per process virtual space, so that even
> though virtual addresses may be found in TLB, their ASID will be different,
> causing TLB miss and probably page fault.
>

My problem is that I want to make self-modified code, so I need to flush both
the instruction and data cache.

>
> >
> > /Carsten
> >
> > --
> > _    _ ____  ___   Carsten Langgaard  Mailto:carstenl@mips.com
> > |\  /|||___)(___   MIPS Denmark        Direct: +45 4486 5527
> > | \/ |||    ____)  Lautrupvang 4B      Switch: +45 4486 5555
> >   TECHNOLOGIES     2750 Ballerup       Fax...: +45 4486 5556
> >                    Denmark            http://www.mips.com
>
> Michael.

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Re: User applications

[Kevin D. Kissell]

> > When a new user process is started will its user space be cleared by the
> > kernel or is there a potential leak from an older user process ?
>
> Usually it is defied by the loader. If the data section contents is set to
> LOAD, then the contents of the section will be loaded from disk (no leak),
> if not -- whatever values left i nmemory will be there, or exactly, the
> virtual page of some other proccess that was swapped out or ended.

Note that what you are describing here is the "exec()" behavior.
I believe Carsten was talking about what happens on a "fork()".

> > What about the registers values, are they cleared for each new user
> > application or will it simply contain the current value it got when the
> > user application is started ?
>
> It depends on the context switch algorithm of the processor, I think.

On a fork() (or presumably clone()) operation, the set of registers
is copied.  Loading a new program ("exec()") should set up the
registers that point to the base of the new stack, the environment,
etc.  Historically, it's up to the runtime startup code ("crt0" in old
Unix systems) to do any other register initialization.

> > How can you flush the data and instruction cashes from a user
> > application ?
> >
>
> As far as I understand, ASID must take care of it. It contains unique IDs
> per process virtual space, so that even
> though virtual addresses may be found in TLB, their ASID will be
different,
> causing TLB miss and probably page fault.

That won't necessarily affect the caches, though.  While it
would be possible to do so, I don't believe any existing
MIPS implementations include ASID in the cache tags.
Hits are determined by an address match, period.

Back in the Ancient Old Days of System V, every architecture
had an architecture-specific system call entry, the first parameter
of which expressed what needed to be done.  Do we have
such a thing in Linux?   That would be the logical place to
things like cache flush and the atomic operations that were
being discussed here a couple of weeks ago.

            Regards,

            Kevin K.

Re: User applications

[Kevin D. Kissell]

> > When a new user process is started will its user space be cleared by the
> > kernel or is there a potential leak from an older user process ?
>
> Usually it is defied by the loader. If the data section contents is set to
> LOAD, then the contents of the section will be loaded from disk (no leak),
> if not -- whatever values left i nmemory will be there, or exactly, the
> virtual page of some other proccess that was swapped out or ended.

Note that what you are describing here is the "exec()" behavior.
I believe Carsten was talking about what happens on a "fork()".

> > What about the registers values, are they cleared for each new user
> > application or will it simply contain the current value it got when the
> > user application is started ?
>
> It depends on the context switch algorithm of the processor, I think.

On a fork() (or presumably clone()) operation, the set of registers
is copied.  Loading a new program ("exec()") should set up the
registers that point to the base of the new stack, the environment,
etc.  Historically, it's up to the runtime startup code ("crt0" in old
Unix systems) to do any other register initialization.

> > How can you flush the data and instruction cashes from a user
> > application ?
> >
>
> As far as I understand, ASID must take care of it. It contains unique IDs
> per process virtual space, so that even
> though virtual addresses may be found in TLB, their ASID will be
different,
> causing TLB miss and probably page fault.

That won't necessarily affect the caches, though.  While it
would be possible to do so, I don't believe any existing
MIPS implementations include ASID in the cache tags.
Hits are determined by an address match, period.

Back in the Ancient Old Days of System V, every architecture
had an architecture-specific system call entry, the first parameter
of which expressed what needed to be done.  Do we have
such a thing in Linux?   That would be the logical place to
things like cache flush and the atomic operations that were
being discussed here a couple of weeks ago.

            Regards,

            Kevin K.

Re: User applications

[Carsten Langgaard]

"Kevin D. Kissell" wrote:

> > > When a new user process is started will its user space be cleared by the
> > > kernel or is there a potential leak from an older user process ?
> >
> > Usually it is defied by the loader. If the data section contents is set to
> > LOAD, then the contents of the section will be loaded from disk (no leak),
> > if not -- whatever values left i nmemory will be there, or exactly, the
> > virtual page of some other proccess that was swapped out or ended.
>
> Note that what you are describing here is the "exec()" behavior.
> I believe Carsten was talking about what happens on a "fork()".
>
> > > What about the registers values, are they cleared for each new user
> > > application or will it simply contain the current value it got when the
> > > user application is started ?
> >
> > It depends on the context switch algorithm of the processor, I think.
>
> On a fork() (or presumably clone()) operation, the set of registers
> is copied.  Loading a new program ("exec()") should set up the
> registers that point to the base of the new stack, the environment,
> etc.  Historically, it's up to the runtime startup code ("crt0" in old
> Unix systems) to do any other register initialization.
>
> > > How can you flush the data and instruction cashes from a user
> > > application ?
> > >
> >
> > As far as I understand, ASID must take care of it. It contains unique IDs
> > per process virtual space, so that even
> > though virtual addresses may be found in TLB, their ASID will be
> different,
> > causing TLB miss and probably page fault.
>
> That won't necessarily affect the caches, though.  While it
> would be possible to do so, I don't believe any existing
> MIPS implementations include ASID in the cache tags.
> Hits are determined by an address match, period.
>
> Back in the Ancient Old Days of System V, every architecture
> had an architecture-specific system call entry, the first parameter
> of which expressed what needed to be done.  Do we have
> such a thing in Linux?   That would be the logical place to
> things like cache flush and the atomic operations that were
> being discussed here a couple of weeks ago.

I think I just found it.
The system call is sysmips(FLUSH_CACHE).

>
>             Regards,
>
>             Kevin K.

--
_    _ ____  ___   Carsten Langgaard   Mailto:carstenl@mips.com
|\  /|||___)(___   MIPS Denmark        Direct: +45 4486 5527
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Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 03:16:16PM +0100, Carsten Langgaard wrote:

> I think I just found it.
> The system call is sysmips(FLUSH_CACHE).

Don't.  Sysmips(FLUSH_CACHE, ...) only allows very coarse flush operation,
that is flushing all caches.  The whole sysmips(2) call exists in Linux
only as a stone age compatibility thing.

  Ralf

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Kevin D. Kissell wrote:

> Back in the Ancient Old Days of System V, every architecture
> had an architecture-specific system call entry, the first parameter
> of which expressed what needed to be done.  Do we have
> such a thing in Linux?   That would be the logical place to
> things like cache flush and the atomic operations that were
> being discussed here a couple of weeks ago.

 The only case caches need to be synchronized is modifying some code.  The
ptrace syscall does it automatically for text writes -- it's needed and
used by gdb to set breakpoints, for example.  For other code there is
cacheflush() which allows you to flush a cache range relevant to a given
virtual address (I see it's not implemented very well at the moment).

 Obviously, you don't want to allow unprivileged users to flush caches as
a whole as it could lead to a DoS. 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Kevin D. Kissell]

----- Original Message -----
From: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>
To: "Kevin D. Kissell" <kevink@mips.com>
Cc: <linux-mips@oss.sgi.com>; "Carsten Langgaard" <carstenl@mips.com>;
"Michael Shmulevich" <michaels@jungo.com>
Sent: Monday, January 08, 2001 4:07 PM
Subject: Re: User applications


> On Mon, 8 Jan 2001, Kevin D. Kissell wrote:
>
> > Back in the Ancient Old Days of System V, every architecture
> > had an architecture-specific system call entry, the first parameter
> > of which expressed what needed to be done.  Do we have
> > such a thing in Linux?   That would be the logical place to
> > things like cache flush and the atomic operations that were
> > being discussed here a couple of weeks ago.
>
>  The only case caches need to be synchronized is modifying some code.

Which just happens to be the case under discussion here.

>The  ptrace syscall does it automatically for text writes -- it's needed
and
> used by gdb to set breakpoints, for example.  For other code there is
> cacheflush() which allows you to flush a cache range relevant to a given
> virtual address (I see it's not implemented very well at the moment).
>
>  Obviously, you don't want to allow unprivileged users to flush caches as
> a whole as it could lead to a DoS.

By that logic, we should not allow users to allocate more virtual
memory than there is physical memory in the system!  A pathological
swap program is arguably far a far worse denial of service attack
than flushing the caches - so long as by "flush" we mean invalidate
with writeback (on copyback caches), of course.

Re: User applications

[Kevin D. Kissell]

----- Original Message -----
From: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>
To: "Kevin D. Kissell" <kevink@mips.com>
Cc: <linux-mips@oss.sgi.com>; "Carsten Langgaard" <carstenl@mips.com>;
"Michael Shmulevich" <michaels@jungo.com>
Sent: Monday, January 08, 2001 4:07 PM
Subject: Re: User applications


> On Mon, 8 Jan 2001, Kevin D. Kissell wrote:
>
> > Back in the Ancient Old Days of System V, every architecture
> > had an architecture-specific system call entry, the first parameter
> > of which expressed what needed to be done.  Do we have
> > such a thing in Linux?   That would be the logical place to
> > things like cache flush and the atomic operations that were
> > being discussed here a couple of weeks ago.
>
>  The only case caches need to be synchronized is modifying some code.

Which just happens to be the case under discussion here.

>The  ptrace syscall does it automatically for text writes -- it's needed
and
> used by gdb to set breakpoints, for example.  For other code there is
> cacheflush() which allows you to flush a cache range relevant to a given
> virtual address (I see it's not implemented very well at the moment).
>
>  Obviously, you don't want to allow unprivileged users to flush caches as
> a whole as it could lead to a DoS.

By that logic, we should not allow users to allocate more virtual
memory than there is physical memory in the system!  A pathological
swap program is arguably far a far worse denial of service attack
than flushing the caches - so long as by "flush" we mean invalidate
with writeback (on copyback caches), of course.

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Kevin D. Kissell wrote:

> >  Obviously, you don't want to allow unprivileged users to flush caches as
> > a whole as it could lead to a DoS.
> 
> By that logic, we should not allow users to allocate more virtual
> memory than there is physical memory in the system!  A pathological
> swap program is arguably far a far worse denial of service attack

 There are limits -- see `info setrlimit'.  There is no way to prevent a
program from executing:

while (1) flush_cache_all();

though but the system's performance would suffer much.  Remember there is
real world out there... 

 Which means sysmips(FLUSH_CACHE, ...) needs to be fixed or removed. 

> than flushing the caches - so long as by "flush" we mean invalidate
> with writeback (on copyback caches), of course.

 What's wrong with cacheflush(addr, count, which) that actually checks if
<addr; addr+count> lies within the caller's address space before
performing the flush and returns -EPERM otherwise?  It would make the
caller crawl like a turtle if it wished to but it would leave other
processes alone. 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 04:40:06PM +0100, Maciej W. Rozycki wrote:

> > than flushing the caches - so long as by "flush" we mean invalidate
> > with writeback (on copyback caches), of course.
> 
>  What's wrong with cacheflush(addr, count, which) that actually checks if
> <addr; addr+count> lies within the caller's address space before
> performing the flush and returns -EPERM otherwise?  It would make the
> caller crawl like a turtle if it wished to but it would leave other
> processes alone. 

cacheflush(2) actually is supposed to handle things that way.

  Ralf

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Ralf Baechle wrote:

> >  What's wrong with cacheflush(addr, count, which) that actually checks if
> > <addr; addr+count> lies within the caller's address space before
> > performing the flush and returns -EPERM otherwise?  It would make the
> > caller crawl like a turtle if it wished to but it would leave other
> > processes alone. 
> 
> cacheflush(2) actually is supposed to handle things that way.

 Didn't I write it clearly-enough? ;-) 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 05:43:24PM +0100, Maciej W. Rozycki wrote:

> > >  What's wrong with cacheflush(addr, count, which) that actually checks if
> > > <addr; addr+count> lies within the caller's address space before
> > > performing the flush and returns -EPERM otherwise?  It would make the
> > > caller crawl like a turtle if it wished to but it would leave other
> > > processes alone. 
> > 
> > cacheflush(2) actually is supposed to handle things that way.
> 
>  Didn't I write it clearly-enough? ;-) 

return -EOVERBLOODINCAFFEIN;

  Ralf

Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 04:07:31PM +0100, Maciej W. Rozycki wrote:

>  The only case caches need to be synchronized is modifying some code.  The
> ptrace syscall does it automatically for text writes -- it's needed and
> used by gdb to set breakpoints, for example.  For other code there is
> cacheflush() which allows you to flush a cache range relevant to a given
> virtual address (I see it's not implemented very well at the moment).
> 
>  Obviously, you don't want to allow unprivileged users to flush caches as
> a whole as it could lead to a DoS. 

You obviously want to allow partial cache flushes or trampolines don't work
and flushing the entire cache can be constructed from that.

  Ralf

Re: User applications

[Carsten Langgaard]

Ralf Baechle wrote:

> On Mon, Jan 08, 2001 at 04:07:31PM +0100, Maciej W. Rozycki wrote:
>
> >  The only case caches need to be synchronized is modifying some code.  The
> > ptrace syscall does it automatically for text writes -- it's needed and
> > used by gdb to set breakpoints, for example.  For other code there is
> > cacheflush() which allows you to flush a cache range relevant to a given
> > virtual address (I see it's not implemented very well at the moment).
> >
> >  Obviously, you don't want to allow unprivileged users to flush caches as
> > a whole as it could lead to a DoS.
>
> You obviously want to allow partial cache flushes or trampolines don't work
> and flushing the entire cache can be constructed from that.
>

What we need is a mechanism to partial invalidate the I-cache and a mechanism
to write-back and/or invalidate the D-cache.

/Carsten

--
_    _ ____  ___   Carsten Langgaard   Mailto:carstenl@mips.com
|\  /|||___)(___   MIPS Denmark        Direct: +45 4486 5527
| \/ |||    ____)  Lautrupvang 4B      Switch: +45 4486 5555
  TECHNOLOGIES     2750 Ballerup       Fax...: +45 4486 5556
                   Denmark             http://www.mips.com

Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 05:23:20PM +0100, Carsten Langgaard wrote:

> What we need is a mechanism to partial invalidate the I-cache and a mechanism
> to write-back and/or invalidate the D-cache.

There is this nice little man page which should even be installed on your
Linux/Inhell box:


CACHEFLUSH(2)       Linux Programmer's Manual       CACHEFLUSH(2)


NAME
       cacheflush  -  flush  contents  of instruction and/or data
       cache

SYNOPSIS
       #include <asm/cachectl.h>

       int cacheflush(char *addr, int nbytes, int cache);

DESCRIPTION
       cacheflush flushes contents of indicated cache(s) for user
       addresses  in the range addr to (addr+nbytes-1). Cache may
       be one of:

       ICACHE Flush the instruction cache.

       DCACHE Write back to memory and  invalidate  the  affected
              valid cache lines.

       BCACHE Same as (ICACHE|DCACHE).


RETURN VALUE
       cacheflush  returns 0 on success or -1 on error. If errors
       are detected, errno will indicate the error.

ERRORS
       EINVAL cache parameter is not one of  ICACHE,  DCACHE,  or
              BCACHE.

       EFAULT Some   or   all   of  the  address  range  addr  to
              (addr+nbytes-1) is not accessible.


BUGS
       The current implementation ignores  the  addr  and  nbytes
       parameters.   Therefore always the whole cache is flushed.

NOTE
       This system call is only available on MIPS based  systems.
       It should not be used in programs intended to be portable.
















Linux 2.0.32                27 June 95                          1

Re: User applications

[Carsten Langgaard]

Exactly that I need, but I don't think it is implemented properly for mips.
It simply flushes all the caches, no matter what options you gives it.

/Carsten

Ralf Baechle wrote:

> On Mon, Jan 08, 2001 at 05:23:20PM +0100, Carsten Langgaard wrote:
>
> > What we need is a mechanism to partial invalidate the I-cache and a mechanism
> > to write-back and/or invalidate the D-cache.
>
> There is this nice little man page which should even be installed on your
> Linux/Inhell box:
>
> CACHEFLUSH(2)       Linux Programmer's Manual       CACHEFLUSH(2)
>
> NAME
>        cacheflush  -  flush  contents  of instruction and/or data
>        cache
>
> SYNOPSIS
>        #include <asm/cachectl.h>
>
>        int cacheflush(char *addr, int nbytes, int cache);
>
> DESCRIPTION
>        cacheflush flushes contents of indicated cache(s) for user
>        addresses  in the range addr to (addr+nbytes-1). Cache may
>        be one of:
>
>        ICACHE Flush the instruction cache.
>
>        DCACHE Write back to memory and  invalidate  the  affected
>               valid cache lines.
>
>        BCACHE Same as (ICACHE|DCACHE).
>
> RETURN VALUE
>        cacheflush  returns 0 on success or -1 on error. If errors
>        are detected, errno will indicate the error.
>
> ERRORS
>        EINVAL cache parameter is not one of  ICACHE,  DCACHE,  or
>               BCACHE.
>
>        EFAULT Some   or   all   of  the  address  range  addr  to
>               (addr+nbytes-1) is not accessible.
>
> BUGS
>        The current implementation ignores  the  addr  and  nbytes
>        parameters.   Therefore always the whole cache is flushed.
>
> NOTE
>        This system call is only available on MIPS based  systems.
>        It should not be used in programs intended to be portable.
>
> Linux 2.0.32                27 June 95                          1

--
_    _ ____  ___   Carsten Langgaard   Mailto:carstenl@mips.com
|\  /|||___)(___   MIPS Denmark        Direct: +45 4486 5527
| \/ |||    ____)  Lautrupvang 4B      Switch: +45 4486 5555
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                   Denmark             http://www.mips.com

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Carsten Langgaard wrote:

> Exactly that I need, but I don't think it is implemented properly for mips.
> It simply flushes all the caches, no matter what options you gives it.

 Yep, it's just a limitation of the implementation.  But it's the right
function and now that I know of the problem it goes to my to do list (but
processing of the list is not that fast these days, so anyone feel free to
do it oneself). 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Carsten Langgaard wrote:

> What we need is a mechanism to partial invalidate the I-cache and a mechanism
> to write-back and/or invalidate the D-cache.

 What's wrong with the already mentioned cachectl?

$ mipsel-linux-objdump -T /usr/mipsel-linux/lib/libc-2.2.so | grep cachectl
00000000600ca0a0  w   DF *ABS*  0000000000000000  GLIBC_2.0   cachectl
$ ls /usr/mipsel-linux/include/sys/cachectl.h
/usr/mipsel-linux/include/sys/cachectl.h

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 05:40:23PM +0100, Maciej W. Rozycki wrote:

> > What we need is a mechanism to partial invalidate the I-cache and a mechanism
> > to write-back and/or invalidate the D-cache.
> 
>  What's wrong with the already mentioned cachectl?
> 
> $ mipsel-linux-objdump -T /usr/mipsel-linux/lib/libc-2.2.so | grep cachectl
> 00000000600ca0a0  w   DF *ABS*  0000000000000000  GLIBC_2.0   cachectl
> $ ls /usr/mipsel-linux/include/sys/cachectl.h
> /usr/mipsel-linux/include/sys/cachectl.h

cachectl(2) is a syscall that is manipulates the cachability of a memory
area.  And not yet implemented ...

  Ralf

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Ralf Baechle wrote:

> > $ mipsel-linux-objdump -T /usr/mipsel-linux/lib/libc-2.2.so | grep cachectl
> > 00000000600ca0a0  w   DF *ABS*  0000000000000000  GLIBC_2.0   cachectl
> > $ ls /usr/mipsel-linux/include/sys/cachectl.h
> > /usr/mipsel-linux/include/sys/cachectl.h
> 
> cachectl(2) is a syscall that is manipulates the cachability of a memory
> area.  And not yet implemented ...

 s/cachectl$/cacheflush/, of course (but the header is still valid).

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Michael Shmulevich]

As a side question, I would like to to know why exactly the CPU cache operations
are
promoted to the syscall status? What is the situation that a user in its program
would like
to call cacheflush() ? Unless, of course, he is doing DoS.

I can understand why we need this in kernel, for context switch, for example, but
as a syscall?...

Michael.

"Maciej W. Rozycki" wrote:

> On Mon, 8 Jan 2001, Ralf Baechle wrote:
>
> > > $ mipsel-linux-objdump -T /usr/mipsel-linux/lib/libc-2.2.so | grep cachectl
> > > 00000000600ca0a0  w   DF *ABS*  0000000000000000  GLIBC_2.0   cachectl
> > > $ ls /usr/mipsel-linux/include/sys/cachectl.h
> > > /usr/mipsel-linux/include/sys/cachectl.h
> >
> > cachectl(2) is a syscall that is manipulates the cachability of a memory
> > area.  And not yet implemented ...
>
>  s/cachectl$/cacheflush/, of course (but the header is still valid).
>
> --
> +  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
> +--------------------------------------------------------------+
> +        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Geert Uytterhoeven]

On Tue, 9 Jan 2001, Michael Shmulevich wrote:
> As a side question, I would like to to know why exactly the CPU cache operations
> are
> promoted to the syscall status? What is the situation that a user in its program
> would like
> to call cacheflush() ? Unless, of course, he is doing DoS.
> 
> I can understand why we need this in kernel, for context switch, for example, but
> as a syscall?...

For trampolines. These are small pieces of code created on the stack, and
require flushing of the caches before they are excuted.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven ------------- Sony Software Development Center Europe (SDCE)
Geert.Uytterhoeven@sonycom.com ------------------- Sint-Stevens-Woluwestraat 55
Voice +32-2-7248626 Fax +32-2-7262686 ---------------- B-1130 Brussels, Belgium

Re: User applications

[Alan Cox]

> promoted to the syscall status? What is the situation that a user in its program
> would like
> to call cacheflush() ? Unless, of course, he is doing DoS.

A cache flush is not a denial of service attack. Its no less effective than a
1Mb memcpy 

> I can understand why we need this in kernel, for context switch, for example, but
> as a syscall?...

Self modifying code, dynamic compilation, glibc trampolines

Re: User applications

[Alan Cox]

> promoted to the syscall status? What is the situation that a user in its program
> would like
> to call cacheflush() ? Unless, of course, he is doing DoS.

A cache flush is not a denial of service attack. Its no less effective than a
1Mb memcpy 

> I can understand why we need this in kernel, for context switch, for example, but
> as a syscall?...

Self modifying code, dynamic compilation, glibc trampolines

Re: User applications

[Maciej W. Rozycki]

On Tue, 9 Jan 2001, Michael Shmulevich wrote:

> As a side question, I would like to to know why exactly the CPU cache operations
> are
> promoted to the syscall status? What is the situation that a user in its program
> would like
> to call cacheflush() ? Unless, of course, he is doing DoS.

 Any software that modifes text needs it.  For example the dynamic linker
or libdl. 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Ralf Baechle wrote:

> You obviously want to allow partial cache flushes or trampolines don't work
> and flushing the entire cache can be constructed from that.

 It depends on the implementation.  The current one obviously allows it. 
;-) 

 I'm writing of the design. 

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Maciej W. Rozycki]

On Mon, 8 Jan 2001, Michael Shmulevich wrote:

> > When a new user process is started will its user space be cleared by the
> > kernel or is there a potential leak from an older user process ?
> 
> Usually it is defied by the loader. If the data section contents is set to
> LOAD, then the contents of the section will be loaded from disk (no leak),
> if not -- whatever values left i nmemory will be there, or exactly, the
> virtual page of some other proccess that was swapped out or ended.

 What!???  I'm assume you are writing about executing a new program and
not forking a new process here.  In the latter case no memory is changed. 
When you exec a new program, any allocated memory is cleared by the kernel
before returning to the user space.  It would be a huge security hole
otherwise.

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

Re: User applications

[Ralf Baechle]

On Mon, Jan 08, 2001 at 10:40:12AM +0100, Carsten Langgaard wrote:

> When a new user process is started will its user space be cleared by the
> kernel or is there a potential leak from an older user process ?

A new process is started by the clone(2) or fork(2) syscalls.  Module the
options that can be passed to clone(2) the two only create an identical copy
of the invoking process, so they're designed to leak information by design ;-)

execve(2) replaces the existing mappings with a new process image loaded
from files plus a newly created stack area.  No old mappings survive, so
there in memory there is no information leak.

> What about the registers values, are they cleared for each new user
> application or will it simply contain the current value it got when the
> user application is started ?

We make no attempt at the integer registers for a new process, so some
information might be leaked in registers.  All the callee saved registers
will be passed unchanged to the child process; the caller saved registers
except those that are used as syscall return values will return random
garbage.  Floating point registers will be cleared with SNANs as soon
as the process is attempting to use a FPU for the first time, that is
we won't leak information via fpu registers.

(Ooops, we're not Orange Book B1 compliant, how sad ;-)

> How can you flush the data and instruction cashes from a user application ?

cacheflush(2).  See man page.

  Ralf