Re: Syscall changes registers beyond %eax, on linux-i386

Re: Syscall changes registers beyond %eax, on linux-i386

[Petr Vandrovec]

On 19 Sep 02 at 13:22, Richard B. Johnson wrote:

> > >>A short snippet of sys_poll, with irrelavant data removed.
> > >>
> > >>sys_poll(struct pollfd *ufds, .. , ..) {
> > >>    ...
> > >>    ufds++;
> > >>    ...
> 
> Well which one?  Here is an ioctl(). It certainly modifies one
> of its parameter values.

poll(), as was already noted. Program below should
print same value for B= and F=, but it reports f + 8*c instead
(where c = number of filedescriptors passed to poll).

And you must call it from assembly, as your calls to getpid() or
ioctl() (or poll()) are wrapped in libc - and glibc's code begins with
push %ebx because of %ebx is used by -fPIC code.

It is questinable whether we should try to not modify parameters
passed into functions. It is definitely nice behavior, but I think
that we should only guarantee that syscalls do not modify unused
registers.
                                                    Petr Vandrovec
                                                    vandrove@vc.cvut.cz
 
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/poll.h>

struct pollfd f[5];

int main(int argc, char* argv[]) {
    unsigned int i;
    void * reg;

    for (i = 0; i < 5; i++) {
        f[i].fd = 0;
        f[i].events = POLLIN;
    }
    __asm__ __volatile__("int $0x80\n" : "=b"(reg) : "a"(168), "0"(f), "c"(5), "d"(1));
    printf("B=%p F=%p\n", reg, f);
    return 0;
}

Re: Syscall changes registers beyond %eax, on linux-i386

[Brian Gerst]

Petr Vandrovec wrote:
> On 19 Sep 02 at 13:22, Richard B. Johnson wrote:
> 
> 
>>>>>A short snippet of sys_poll, with irrelavant data removed.
>>>>>
>>>>>sys_poll(struct pollfd *ufds, .. , ..) {
>>>>>   ...
>>>>>   ufds++;
>>>>>   ...
>>>>
>>Well which one?  Here is an ioctl(). It certainly modifies one
>>of its parameter values.
> 
> 
> poll(), as was already noted. Program below should
> print same value for B= and F=, but it reports f + 8*c instead
> (where c = number of filedescriptors passed to poll).
> 
> And you must call it from assembly, as your calls to getpid() or
> ioctl() (or poll()) are wrapped in libc - and glibc's code begins with
> push %ebx because of %ebx is used by -fPIC code.
> 
> It is questinable whether we should try to not modify parameters
> passed into functions. It is definitely nice behavior, but I think
> that we should only guarantee that syscalls do not modify unused
> registers.
>                                                     Petr Vandrovec
>                                                     vandrove@vc.cvut.cz

Now that I've thought about it more, I think the best solution is to go 
through all the syscalls (a big job, I know), and declare the parameters 
as const, so that gcc knows it can't modify them, and will throw a 
warning if we try.

--
				Brian Gerst

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard Henderson]

On Thu, Sep 19, 2002 at 02:04:43PM -0400, Brian Gerst wrote:
> Now that I've thought about it more, I think the best solution is to go 
> through all the syscalls (a big job, I know), and declare the parameters 
> as const, so that gcc knows it can't modify them, and will throw a 
> warning if we try.

The parameter area belongs to the callee, and it may *always* be modified.


r~

Re: Syscall changes registers beyond %eax, on linux-i386

[Brian Gerst]

Richard Henderson wrote:
> On Thu, Sep 19, 2002 at 02:04:43PM -0400, Brian Gerst wrote:
> 
>>Now that I've thought about it more, I think the best solution is to go 
>>through all the syscalls (a big job, I know), and declare the parameters 
>>as const, so that gcc knows it can't modify them, and will throw a 
>>warning if we try.
> 
> 
> The parameter area belongs to the callee, and it may *always* be modified.
> 
> 
> r~
> 

The parameters can not be modified if they are declared const though, 
that's my point.

--
				Brian Gerst

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard Henderson]

On Thu, Sep 19, 2002 at 02:51:44PM -0400, Brian Gerst wrote:
> > The parameter area belongs to the callee, and it may *always* be modified.
> 
> The parameters can not be modified if they are declared const though, 
> that's my point.

Yes they can.

	extern void bar(int x, int y, int z);
	void foo(const int a, const int b, const int c)
	{
	  bar(a+1, b+1, c+1);
	}

        subl    $12, %esp
        movl    20(%esp), %eax
        incl    %eax
        movl    %eax, 20(%esp)
        movl    16(%esp), %eax
        incl    %eax
        incl    24(%esp)
        movl    %eax, 16(%esp)
        addl    $12, %esp
        jmp     bar

(Not sure why gcc doesn't use incl on all three memories, nor
should it allocate that stack frame...)


r~

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard B. Johnson]

On Thu, 19 Sep 2002, Richard Henderson wrote:

> On Thu, Sep 19, 2002 at 02:51:44PM -0400, Brian Gerst wrote:
> > > The parameter area belongs to the callee, and it may *always* be modified.
> > 
> > The parameters can not be modified if they are declared const though, 
> > that's my point.
> 
> Yes they can.
> 
> 	extern void bar(int x, int y, int z);
> 	void foo(const int a, const int b, const int c)
> 	{
> 	  bar(a+1, b+1, c+1);
> 	}
> 
>         subl    $12, %esp
>         movl    20(%esp), %eax
>         incl    %eax
>         movl    %eax, 20(%esp)
>         movl    16(%esp), %eax
>         incl    %eax
>         incl    24(%esp)
>         movl    %eax, 16(%esp)
>         addl    $12, %esp
>         jmp     bar
> 
> (Not sure why gcc doesn't use incl on all three memories, nor
> should it allocate that stack frame...)
> 
> 
> r~
> 

Well it's not modifying those values. It's putting the
constant value into a register and modifying the value
in the register before calling a function that takes int.
Note that the parameter passed to the function, a, b, and c,
are local copies. gcc can whack those anyway it wants. In
fact, it does strange things above which may not be valid.
It subtracts an offset from esp for local variables ($12).
There aren't any local variables!. Therefore, it has to
access the passed parameters at their pushed offset + 12.
Then, after it's through mucking with them, it collapses
the local stack area (levels the stack), then jumps
to the called function. It will use the early 'call'
return-value to return to the caller.
It's really bad code because it could have done:

	incl	$0x04(%esp)
	incl	$0x08(%esp)
	incl	$0x1c(%esp)
	jmp	bar

Note that, in every case, the constant value was pushed onto the
stack and this function called. That copy of the constant value
can be trashed anyway the callee wants. It's his copy.


I thought you were going to do something like:

Script started on Thu Sep 19 15:22:05 2002
# cat zzz.c

int foo(const int a, const int b, const int c)
{
    a += b;
    a += c;
    return a;
}
# gcc -c -o zzz zzz.c
zzz.c: In function `foo':
zzz.c:6: warning: assignment of read-only location
zzz.c:7: warning: assignment of read-only location
# exit
exit

Script done on Thu Sep 19 15:22:23 2002

Which makes gcc barf when you attempt to modify the
const value. This allows you to check if the code is
doing the wrong thing.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
Yes, top management were graduates of West Point and Annapolis.

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard Henderson]

On Thu, Sep 19, 2002 at 03:40:52PM -0400, Richard B. Johnson wrote:
> Well it's not modifying those values.

It's not modifying "a", true, but it _is_ modifying the parameter
area.  Which is exactly the kernel bug in question.

> It's really bad code because it could have done:
> 
> 	incl	$0x04(%esp)
> 	incl	$0x08(%esp)
> 	incl	$0x1c(%esp)
> 	jmp	bar

Yes, I know.


r~

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard B. Johnson]

On Thu, 19 Sep 2002, Richard Henderson wrote:

> On Thu, Sep 19, 2002 at 03:40:52PM -0400, Richard B. Johnson wrote:
> > Well it's not modifying those values.
> 
> It's not modifying "a", true, but it _is_ modifying the parameter
> area.  Which is exactly the kernel bug in question.
> 

Yep. This can't be found by the compiler. The parameter area is
writable so it looks like somebody needs to do some 'code inspection'
and some additional testing.

> > It's really bad code because it could have done:
> > 
> > 	incl	$0x04(%esp)
> > 	incl	$0x08(%esp)
> > 	incl	$0x1c(%esp)
> > 	jmp	bar
> 
> Yes, I know.
> 

It's a problem with a 'general purpose' compiler that wants to
be "all things" to all people. If somebody made a gcc-compatible
compiler, tuned to the ix86 characteristics, I think we could
cut the extra instructions by at least 1/2, maybe more.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
Yes, top management were graduates of West Point and Annapolis.

Re: Syscall changes registers beyond %eax, on linux-i386

[J.A. Magallon]

On 2002.09.19 Richard B. Johnson wrote:
>On Thu, 19 Sep 2002, Richard Henderson wrote:
>
[...]
>> > It's really bad code because it could have done:
>> > 
>> > 	incl	$0x04(%esp)
>> > 	incl	$0x08(%esp)
>> > 	incl	$0x1c(%esp)
>> > 	jmp	bar
>> 
[...]
>
>It's a problem with a 'general purpose' compiler that wants to
>be "all things" to all people. If somebody made a gcc-compatible
>compiler, tuned to the ix86 characteristics, I think we could
>cut the extra instructions by at least 1/2, maybe more.
>

Curiosity killed the cat....
Just tried it with gcc-3.2.
C code:
extern void bar(int x, int y, int z);
void foo(const int a, const int b, const int c)
{
        bar(a+1, b+1, c+1);
}

- gcc -S -O0:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        subl    $4, %esp
        movl    16(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    12(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    8(%ebp), %eax
        incl    %eax
        pushl   %eax
        call    bar
        addl    $16, %esp
        leave
        ret

- gcc -S -O1:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $12, %esp
        movl    16(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    12(%ebp), %eax
        incl    %eax
        pushl   %eax
        movl    8(%ebp), %eax
        incl    %eax
        pushl   %eax
        call    bar
        addl    $16, %esp
        movl    %ebp, %esp
        popl    %ebp
        ret

- gcc -S -O2:
        movl    12(%esp), %eax
        incl    %eax
        movl    %eax, 12(%esp)
        movl    8(%esp), %eax
        incl    %eax
        movl    %eax, 8(%esp)
        movl    4(%esp), %eax
        incl    %eax
        movl    %eax, 4(%esp)
        jmp     bar

- gcc -S -O2 -march=[i686,pentium2,pentium3]:
        incl    4(%esp)
        movl    8(%esp), %eax
        incl    %eax
        movl    %eax, 8(%esp)
        movl    12(%esp), %eax
        incl    %eax
        movl    %eax, 12(%esp)
        jmp     bar

- gcc -S -O2 -march=pentium4:
        movl    8(%esp), %eax
        addl    $1, 4(%esp)
        addl    $1, %eax
        movl    %eax, 8(%esp)
        movl    12(%esp), %eax
        addl    $1, %eax
        movl    %eax, 12(%esp)
        jmp     bar

-- 
J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
werewolf.able.es                         \           It's better when it's free
Mandrake Linux release 9.0 (Cooker) for i586
Linux 2.4.20-pre7-jam0 (gcc 3.2 (Mandrake Linux 9.0 3.2-1mdk))

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard B. Johnson]

On Fri, 20 Sep 2002, J.A. Magallon wrote:

> 
> On 2002.09.19 Richard B. Johnson wrote:
> >On Thu, 19 Sep 2002, Richard Henderson wrote:
> >
> [...]
> >> > It's really bad code because it could have done:
> >> > 
> >> > 	incl	$0x04(%esp)
> >> > 	incl	$0x08(%esp)
> >> > 	incl	$0x1c(%esp)
> >> > 	jmp	bar
> >> 
> [...]
> >
> >It's a problem with a 'general purpose' compiler that wants to
> >be "all things" to all people. If somebody made a gcc-compatible
> >compiler, tuned to the ix86 characteristics, I think we could
> >cut the extra instructions by at least 1/2, maybe more.
> >
> 
> Curiosity killed the cat....
> Just tried it with gcc-3.2.
> C code:
> extern void bar(int x, int y, int z);
> void foo(const int a, const int b, const int c)
> {
>         bar(a+1, b+1, c+1);
> }
> 
> - gcc -S -O0:
>         pushl   %ebp
>         movl    %esp, %ebp
>         subl    $8, %esp
>         subl    $4, %esp
>         movl    16(%ebp), %eax
>         incl    %eax
>         pushl   %eax
>         movl    12(%ebp), %eax
>         incl    %eax
>         pushl   %eax
>         movl    8(%ebp), %eax
>         incl    %eax
>         pushl   %eax
>         call    bar
>         addl    $16, %esp
>         leave
>         ret
> 
> - gcc -S -O1:
>         pushl   %ebp
>         movl    %esp, %ebp
>         subl    $12, %esp
>         movl    16(%ebp), %eax
>         incl    %eax
>         pushl   %eax
>         movl    12(%ebp), %eax
>         incl    %eax
>         pushl   %eax
>         movl    8(%ebp), %eax
>         incl    %eax
>         pushl   %eax
>         call    bar
>         addl    $16, %esp
>         movl    %ebp, %esp
>         popl    %ebp
>         ret
> 
> - gcc -S -O2:
>         movl    12(%esp), %eax
>         incl    %eax
>         movl    %eax, 12(%esp)
>         movl    8(%esp), %eax
>         incl    %eax
>         movl    %eax, 8(%esp)
>         movl    4(%esp), %eax
>         incl    %eax
>         movl    %eax, 4(%esp)
>         jmp     bar
> 
> - gcc -S -O2 -march=[i686,pentium2,pentium3]:
>         incl    4(%esp)
>         movl    8(%esp), %eax
>         incl    %eax
>         movl    %eax, 8(%esp)
>         movl    12(%esp), %eax
>         incl    %eax
>         movl    %eax, 12(%esp)
>         jmp     bar
> 
> - gcc -S -O2 -march=pentium4:
>         movl    8(%esp), %eax
>         addl    $1, 4(%esp)
>         addl    $1, %eax
>         movl    %eax, 8(%esp)
>         movl    12(%esp), %eax
>         addl    $1, %eax
>         movl    %eax, 12(%esp)
>         jmp     bar
> 
> -- 
> J.A. Magallon <jamagallon@able.es>      \                 Software is like sex:
> werewolf.able.es                         \           It's better when it's free
> Mandrake Linux release 9.0 (Cooker) for i586
> Linux 2.4.20-pre7-jam0 (gcc 3.2 (Mandrake Linux 9.0 3.2-1mdk))
> 

Notice that it always gets some value from memory, modifies it,
then writes it back. Adding 1 to %eax is plain dumb. Those instructions
have to be fetched! Any instruction that's longer than the constant
long-word in that instruction should be reviewed. Also that 1 is
4 bytes long. It has a single-byte oprand. That means the next instruction
fetch will be at an odd address if it started on even because that
sequence is 5 bytes in length.


.if 0
	 You can assemble this directly .....

You know there are continuous complaints about
ix86 processors being "register starved", but somehow
the 'C' compilers often don't use the capabilities that
are available with the processors. The following is some
'code' that will assemble. It doesn't do anything useful,
but shows some addressing capability that is often ignored.
.endif


foo:	.long	0

bar:	incl	(foo)		# Bump the value of foo directly
	addl	%eax,(foo)	# Add eax to value in foo
	addl	$0x10,(foo)	# Add constant to value in foo
	addl	(foo),%eax	# Add value in foo to eax
	pushl	(foo)		# Put value in foo onto stack
	popl	(foo)		# Pop value on stack into foo
	movl	%eax, foo(%ebx)	# Put eax value into memory at foo + ebx
	incb	(foo)		# This is atomic, no lock required
	movl	14(%esp, %ebx), %eax # Get value from stack at offset
                                     # ESP + EBX (good for local arrays)

.if 0

Most of the gcc code that deals with memory oprands, gets a value
from memory, modifies it, then writes it back. This is a "throw-back"
from processors that only have load and store operations. The ix86
processors can directly modify a single bit, anywhere in memory without
having to put it into a register. Of course, what the hardware 
physically does may be quite another thing altogether. But I suggest
that the CPU/Hardware combination is more capable of doing the right
thing in executing the binary than any compiler that forces a load
into a register, modification of register contents, then a write
back to memory.

Timing tests with rdtsc show many cycles are often wasted with these forced
load and store operations.

.endif

	

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
Yes, top management were graduates of West Point and Annapolis.

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard Henderson]

On Fri, Sep 20, 2002 at 08:27:32AM -0400, Richard B. Johnson wrote:
> Adding 1 to %eax is plain dumb.

No it isn't.  P4 has a partial register stall on the
flags register when using incl.  You'll notice that
we *do* use incl except when optimizing for P4.

> Also that 1 is 4 bytes long.

No it isn't.  There is an 8-bit signed immediate form.

As for the rest of the memory operand rant, the problem
is not that gcc won't try to use memory operands, it's
that the bit of code that's supposed to put these 
memory operands back together is like 10 years old and
hasn't been taught about the memory aliasing subsystem.
So any time it sees a memory load cross a memory store,
it gives up.

Perhaps I'll have this fixed for gcc 3.4.



r~

Re: Syscall changes registers beyond %eax, on linux-i386

[Pavel Machek]

Hi!

> It's a problem with a 'general purpose' compiler that wants to
> be "all things" to all people. If somebody made a gcc-compatible
> compiler, tuned to the ix86 characteristics, I think we could
> cut the extra instructions by at least 1/2, maybe more.

Remember pgcc? 

And btw cutting instructions by 1/2might look nice but unless you can
keep it as fast as it was, its useless.
								Pavel
-- 
Philips Velo 1: 1"x4"x8", 300gram, 60, 12MB, 40bogomips, linux, mutt,
details at http://atrey.karlin.mff.cuni.cz/~pavel/velo/index.html.

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard B. Johnson]

On Sun, 22 Sep 2002, Pavel Machek wrote:

> Hi!
> 
> > It's a problem with a 'general purpose' compiler that wants to
> > be "all things" to all people. If somebody made a gcc-compatible
> > compiler, tuned to the ix86 characteristics, I think we could
> > cut the extra instructions by at least 1/2, maybe more.
> 
> Remember pgcc? 
> 
> And btw cutting instructions by 1/2might look nice but unless you can
> keep it as fast as it was, its useless.
> 								Pavel
> -- 
Yes, but to see the affect of cutting down the instruction length, you
need to make benchmarks that emulate running 'forever'. Many bench-
marks access some memory over-and-over again in a loop. This does
not exercise the need to refill prefetch so the benchmarks ignore
the advantages obtained by reducing the amount of instructions needed
to be fetched from memory.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
Yes, top management were graduates of West Point and Annapolis.

Re: Syscall changes registers beyond %eax, on linux-i386

[Pavel Machek]

Hi!

> > > It's a problem with a 'general purpose' compiler that wants to
> > > be "all things" to all people. If somebody made a gcc-compatible
> > > compiler, tuned to the ix86 characteristics, I think we could
> > > cut the extra instructions by at least 1/2, maybe more.
> > 
> > Remember pgcc? 
> > 
> > And btw cutting instructions by 1/2might look nice but unless you can
> > keep it as fast as it was, its useless.
> > 								Pavel
> > -- 
> Yes, but to see the affect of cutting down the instruction length, you
> need to make benchmarks that emulate running 'forever'. Many bench-

Specs contain things like perl and gcc, those are I believe far too
big to be put entirely into cache and emulate "Real Life" quite
well...

								Pavel
-- 
Casualities in World Trade Center: ~3k dead inside the building,
cryptography in U.S.A. and free speech in Czech Republic.

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard B. Johnson]

On Thu, 19 Sep 2002, Brian Gerst wrote:

> Richard Henderson wrote:
> > On Thu, Sep 19, 2002 at 02:04:43PM -0400, Brian Gerst wrote:
> > 
> >>Now that I've thought about it more, I think the best solution is to go 
> >>through all the syscalls (a big job, I know), and declare the parameters 
> >>as const, so that gcc knows it can't modify them, and will throw a 
> >>warning if we try.
> > 
> > 
> > The parameter area belongs to the callee, and it may *always* be modified.
> > 
> > 
> > r~
> > 
> 
> The parameters can not be modified if they are declared const though, 
> that's my point.

Yes. A temporary declaration change to compile the kernel and
see where it complains.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
Yes, top management were graduates of West Point and Annapolis.

Re: Syscall changes registers beyond %eax, on linux-i386

[Daniel Jacobowitz]

On Thu, Sep 19, 2002 at 02:04:43PM -0400, Brian Gerst wrote:
> Petr Vandrovec wrote:
> >On 19 Sep 02 at 13:22, Richard B. Johnson wrote:
> >
> >
> >>>>>A short snippet of sys_poll, with irrelavant data removed.
> >>>>>
> >>>>>sys_poll(struct pollfd *ufds, .. , ..) {
> >>>>>  ...
> >>>>>  ufds++;
> >>>>>  ...
> >>>>
> >>Well which one?  Here is an ioctl(). It certainly modifies one
> >>of its parameter values.
> >
> >
> >poll(), as was already noted. Program below should
> >print same value for B= and F=, but it reports f + 8*c instead
> >(where c = number of filedescriptors passed to poll).
> >
> >And you must call it from assembly, as your calls to getpid() or
> >ioctl() (or poll()) are wrapped in libc - and glibc's code begins with
> >push %ebx because of %ebx is used by -fPIC code.
> >
> >It is questinable whether we should try to not modify parameters
> >passed into functions. It is definitely nice behavior, but I think
> >that we should only guarantee that syscalls do not modify unused
> >registers.
> >                                                    Petr Vandrovec
> >                                                    vandrove@vc.cvut.cz
> 
> Now that I've thought about it more, I think the best solution is to go 
> through all the syscalls (a big job, I know), and declare the parameters 
> as const, so that gcc knows it can't modify them, and will throw a 
> warning if we try.

That's not going to help.  As Richard said, the memory in question
belongs to the called function.  GCC knows this.  It can freely modify
it.  The fact that the value of the parameter is const is a
language-level, semantic thing.  It doesn't say anything about the
const-ness of that memory.  Only the ABI does.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: Syscall changes registers beyond %eax, on linux-i386

[Mikael Pettersson]

Daniel Jacobowitz writes:
 > That's not going to help.  As Richard said, the memory in question
 > belongs to the called function.  GCC knows this.  It can freely modify
 > it.  The fact that the value of the parameter is const is a
 > language-level, semantic thing.  It doesn't say anything about the
 > const-ness of that memory.  Only the ABI does.

Does Linux/x86 even have a proper ABI document? I've never seen one.
The closest I've seen would be the SVR4 i386 psABI, but it
deliberately doesn't define the raw syscall interface, only the
each-syscall-is-a-C-function one implemented by the C library,
and that interface doesn't suffer from the current issue.

IOW, the kernel may not be at fault if user-space code invokes int
$0x80 directly and then sees clobbered registers.

/Mikael

Re: Syscall changes registers beyond %eax, on linux-i386

[george anzinger]

Mikael Pettersson wrote:
> 
> Daniel Jacobowitz writes:
>  > That's not going to help.  As Richard said, the memory in question
>  > belongs to the called function.  GCC knows this.  It can freely modify
>  > it.  The fact that the value of the parameter is const is a
>  > language-level, semantic thing.  It doesn't say anything about the
>  > const-ness of that memory.  Only the ABI does.
> 
> Does Linux/x86 even have a proper ABI document? I've never seen one.
> The closest I've seen would be the SVR4 i386 psABI, but it
> deliberately doesn't define the raw syscall interface, only the
> each-syscall-is-a-C-function one implemented by the C library,
> and that interface doesn't suffer from the current issue.
> 
> IOW, the kernel may not be at fault if user-space code invokes int
> $0x80 directly and then sees clobbered registers.

Ah, that, indeed is the issue.  As far as C is concerned,
the call is NOT a call, but a bit of asm.  If the asm is
correctly written the problem goes away, not because the
register is not modified, but because C is on notice that it
MIGHT be modified and thus not to count on it.

As a practical matter, ebx is used to pass arg1 to the
kernel so it must be changed by the asm code, the further
listing of it beyond the third ":" in the asm inline, will
cause the compiler to not rely on it being further
modified.  The same is true of all the registers used to
pass parameters.  (These are: arg1 ebx, arg2 ecx, arg3 edx,
arg4 esi, arg5 edi, and arg6 ebp.)

So, is there a problem?  Yes, neither the call stub macros
in asm/unistd.h nor those in glibc bother to list the used
registers beyond the third ":".  And, if I understand this
right, the glibc code to save ebx in another register
suffers from the false assumption that THAT register can be
clobbered, but this is only true if C sees the code as a
function, not an inline asm, but most system calls in glibc
are coded as inline asm, not separate functions (not to be
confused with the C inline, which is a separate function).

At least that is how I see it.  Comments?

-g

> 
> /Mikael
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-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard Henderson]

On Fri, Sep 20, 2002 at 01:32:05AM -0700, george anzinger wrote:
> So, is there a problem?  Yes, neither the call stub macros
> in asm/unistd.h nor those in glibc bother to list the used
> registers beyond the third ":".

No, this is not the real problem.  The real problem is that if
the program receives a signal during a system call, the kernel
will return all the way up to entry.S, deliver the signal and
then restart the syscall.

Except the syscall will restart with the corrupted registers.

Hilarity ensues.



r~

Re: Syscall changes registers beyond %eax, on linux-i386

[george anzinger]

Richard Henderson wrote:
> 
> On Fri, Sep 20, 2002 at 01:32:05AM -0700, george anzinger wrote:
> > So, is there a problem?  Yes, neither the call stub macros
> > in asm/unistd.h nor those in glibc bother to list the used
> > registers beyond the third ":".
> 
> No, this is not the real problem.  The real problem is that if
> the program receives a signal during a system call, the kernel
> will return all the way up to entry.S, deliver the signal and
> then restart the syscall.
> 
> Except the syscall will restart with the corrupted registers.
> 
> Hilarity ensues.
> 
I submit that BOTH of these are problems.  And only  the
kernel can fix the latter.

-g
-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml

Re: Syscall changes registers beyond %eax, on linux-i386

[Richard Henderson]

On Sat, Sep 21, 2002 at 01:09:12AM -0700, george anzinger wrote:
> > Except the syscall will restart with the corrupted registers.
> > 
> > Hilarity ensues.
> > 
> I submit that BOTH of these are problems.  And only  the
> kernel can fix the latter.

If the later is fixed, so is the former.


r~

CHECKER bate: Syscall changes registers beyond %eax, on linux-i386

[george anzinger]

george anzinger wrote:
> 
> Richard Henderson wrote:
> >
> > On Fri, Sep 20, 2002 at 01:32:05AM -0700, george anzinger wrote:
> > > So, is there a problem?  Yes, neither the call stub macros
> > > in asm/unistd.h nor those in glibc bother to list the used
> > > registers beyond the third ":".
> >
> > No, this is not the real problem.  The real problem is that if
> > the program receives a signal during a system call, the kernel
> > will return all the way up to entry.S, deliver the signal and
> > then restart the syscall.
> >
> > Except the syscall will restart with the corrupted registers.
> >
> > Hilarity ensues.
> >
> I submit that BOTH of these are problems.  And only  the
> kernel can fix the latter.
> 
Sounds like a job for the CHECKER.  Should be easy to verify
that a system call does not modify its call parameters.


-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml