Re: [PATCH 3/4] UML - Properly invoke x86_64 system calls

[linux-os (Dick Johnson)]

On Thu, 17 Nov 2005, Jeff Dike wrote:

> This patch makes stub_segv use the stub_syscall macros.  This was
> needed anyway, but the bug that prompted this was the discovery that
> gcc was storing stuff in RCX, which is trashed across a system
> call.

But the C-calling convention used by gcc for 32-bit systems
is supposed to allow the called-function to destroy general-
purpose registers but not index registers. In other words,
ECX, EDX are supposed to be available and EAX is used for
return-values. The register size isn't supposed to have
anything to do with it (longword/quadword)!

In 64-bit world, the same is supposed to apply as well.
If RCX is now precious, it's a GCC bug that should be fixed.

Yes?

   This is exactly the sort of problem that the new macros fix.
> There is a stub_syscall0 for getpid.
> stub_segv was changed to be a libc file, and that caused some
> include changes.
>
> Signed-off-by: Jeff Dike <jdike@xxxxxxxxxxx>
>
> Index: linux-2.6.15/arch/um/include/sysdep-i386/stub.h
> ===================================================================
> --- linux-2.6.15.orig/arch/um/include/sysdep-i386/stub.h	2005-11-17 14:59:30.000000000 -0500
> +++ linux-2.6.15/arch/um/include/sysdep-i386/stub.h	2005-11-17 14:59:55.000000000 -0500
> @@ -16,6 +16,15 @@
> #define STUB_MMAP_NR __NR_mmap2
> #define MMAP_OFFSET(o) ((o) >> PAGE_SHIFT)
>
> +static inline long stub_syscall0(long syscall)
> +{
> +	long ret;
> +
> +	__asm__ volatile ("int $0x80" : "=a" (ret) : "0" (syscall));
> +
> +	return ret;
> +}
> +
> static inline long stub_syscall1(long syscall, long arg1)
> {
> 	long ret;
> Index: linux-2.6.15/arch/um/include/sysdep-x86_64/stub.h
> ===================================================================
> --- linux-2.6.15.orig/arch/um/include/sysdep-x86_64/stub.h	2005-11-17 14:59:30.000000000 -0500
> +++ linux-2.6.15/arch/um/include/sysdep-x86_64/stub.h	2005-11-17 14:59:55.000000000 -0500
> @@ -6,7 +6,6 @@
> #ifndef __SYSDEP_STUB_H
> #define __SYSDEP_STUB_H
>
> -#include <asm/ptrace.h>
> #include <asm/unistd.h>
> #include <sysdep/ptrace_user.h>
>
> @@ -20,6 +19,17 @@
> #define __syscall_clobber "r11","rcx","memory"
> #define __syscall "syscall"
>
> +static inline long stub_syscall0(long syscall)
> +{
> +	long ret;
> +
> +	__asm__ volatile (__syscall
> +		: "=a" (ret)
> +		: "0" (syscall) : __syscall_clobber );
> +
> +	return ret;
> +}
> +
> static inline long stub_syscall2(long syscall, long arg1, long arg2)
> {
> 	long ret;
> Index: linux-2.6.15/arch/um/sys-i386/Makefile
> ===================================================================
> --- linux-2.6.15.orig/arch/um/sys-i386/Makefile	2005-11-17 14:59:30.000000000 -0500
> +++ linux-2.6.15/arch/um/sys-i386/Makefile	2005-11-17 14:59:55.000000000 -0500
> @@ -5,7 +5,7 @@
> obj-$(CONFIG_HIGHMEM) += highmem.o
> obj-$(CONFIG_MODULES) += module.o
>
> -USER_OBJS := bugs.o ptrace_user.o sigcontext.o fault.o
> +USER_OBJS := bugs.o ptrace_user.o sigcontext.o fault.o stub_segv.o
>
> SYMLINKS = bitops.c semaphore.c highmem.c module.c
>
> Index: linux-2.6.15/arch/um/sys-i386/stub_segv.c
> ===================================================================
> --- linux-2.6.15.orig/arch/um/sys-i386/stub_segv.c	2005-11-17 14:59:30.000000000 -0500
> +++ linux-2.6.15/arch/um/sys-i386/stub_segv.c	2005-11-17 14:59:55.000000000 -0500
> @@ -3,9 +3,11 @@
>  * Licensed under the GPL
>  */
>
> -#include <asm/signal.h>
> +#include <signal.h>
> +#include <sys/select.h> /* The only way I can see to get sigset_t */
> #include <asm/unistd.h>
> #include "uml-config.h"
> +#include "sysdep/stub.h"
> #include "sysdep/sigcontext.h"
> #include "sysdep/faultinfo.h"
>
> @@ -13,13 +15,14 @@
> stub_segv_handler(int sig)
> {
> 	struct sigcontext *sc = (struct sigcontext *) (&sig + 1);
> +	int pid;
>
> 	GET_FAULTINFO_FROM_SC(*((struct faultinfo *) UML_CONFIG_STUB_DATA),
> 			      sc);
>
> -	__asm__("movl %0, %%eax ; int $0x80": : "g" (__NR_getpid));
> -	__asm__("movl %%eax, %%ebx ; movl %0, %%eax ; movl %1, %%ecx ;"
> -		"int $0x80": : "g" (__NR_kill), "g" (SIGUSR1));
> +	pid = stub_syscall0(__NR_getpid);
> +	stub_syscall2(__NR_kill, pid, SIGUSR1);
> +
> 	/* Load pointer to sigcontext into esp, since we need to leave
> 	 * the stack in its original form when we do the sigreturn here, by
> 	 * hand.
> Index: linux-2.6.15/arch/um/sys-x86_64/Makefile
> ===================================================================
> --- linux-2.6.15.orig/arch/um/sys-x86_64/Makefile	2005-11-17 14:59:30.000000000 -0500
> +++ linux-2.6.15/arch/um/sys-x86_64/Makefile	2005-11-17 14:59:55.000000000 -0500
> @@ -12,7 +12,7 @@
> obj-y := ksyms.o
> obj-$(CONFIG_MODULES) += module.o um_module.o
>
> -USER_OBJS := ptrace_user.o sigcontext.o
> +USER_OBJS := ptrace_user.o sigcontext.o stub_segv.o
>
> SYMLINKS = bitops.c csum-copy.S csum-partial.c csum-wrappers.c ldt.c memcpy.S \
> 	thunk.S module.c
> Index: linux-2.6.15/arch/um/sys-x86_64/stub_segv.c
> ===================================================================
> --- linux-2.6.15.orig/arch/um/sys-x86_64/stub_segv.c	2005-11-17 14:59:30.000000000 -0500
> +++ linux-2.6.15/arch/um/sys-x86_64/stub_segv.c	2005-11-17 15:12:23.000000000 -0500
> @@ -3,14 +3,14 @@
>  * Licensed under the GPL
>  */
>
> -#include <asm/signal.h>
> +#include <stddef.h>
> +#include <signal.h>
> #include <linux/compiler.h>
> #include <asm/unistd.h>
> -#include <asm/ucontext.h>
> #include "uml-config.h"
> #include "sysdep/sigcontext.h"
> #include "sysdep/faultinfo.h"
> -#include <stddef.h>
> +#include "sysdep/stub.h"
>
> /* Copied from sys-x86_64/signal.c - Can't find an equivalent definition
>  * in the libc headers anywhere.
> @@ -31,21 +31,21 @@
> stub_segv_handler(int sig)
> {
> 	struct ucontext *uc;
> +        int pid;
>
> 	__asm__("movq %%rdx, %0" : "=g" (uc) :);
> 	GET_FAULTINFO_FROM_SC(*((struct faultinfo *) UML_CONFIG_STUB_DATA),
> 			      &uc->uc_mcontext);
>
> -	__asm__("movq %0, %%rax ; syscall": : "g" (__NR_getpid));
> -	__asm__("movq %%rax, %%rdi ; movq %0, %%rax ; movq %1, %%rsi ;"
> -		"syscall": : "g" (__NR_kill), "g" (SIGUSR1) :
> -		"%rdi", "%rax", "%rsi");
> +	pid = stub_syscall0(__NR_getpid);
> +	stub_syscall2(__NR_kill, pid, SIGUSR1);
> +
> 	/* sys_sigreturn expects that the stack pointer will be 8 bytes into
> 	 * the signal frame.  So, we use the ucontext pointer, which we know
> 	 * already, to get the signal frame pointer, and add 8 to that.
> 	 */
> -	__asm__("movq %0, %%rsp": :
> +	__asm__("movq %0, %%rsp; movq %1, %%rax ; syscall": :
> 		"g" ((unsigned long) container_of(uc, struct rt_sigframe,
> -						  uc) + 8));
> -	__asm__("movq %0, %%rax ; syscall" : : "g" (__NR_rt_sigreturn));
> +						  uc) + 8),
> +                "g" (__NR_rt_sigreturn));
> }
>
> -
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.44 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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