[PATCH v6 14/19] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

Yury Norov
2015-12-01T23:35:03Z

On Tue, Dec 01, 2015 at 11:39:42PM +0100, Arnd Bergmann wrote:
> On Wednesday 02 December 2015 00:29:04 Yury Norov wrote:
> > > > +#define compat_sys_shmat sys_shmat
> > >
> > > What's special about compat_sys_shmat?
> > >
> >
> > It's about definition.
> > For aarch32 glibc defines it as (__getpagesize () << 2).
> > For ILP32 there's no definition, and so generic one is used: (__getpagesize ()).
>
> Ok, got it.
>
> > In kernel, for ARM64, COMPAT_SHMLBA defined just as 0x4000. Both
> > compat and non-compat shmat syscalls pass identical arguments to
> > do_shmat, except shmlba. Effectively, library expects shmlba to
> > be 0x1000, as sys_shmat does. And compat_sys_shmat expects 0x4000.
> >
> > I think, both kernel and library parts are to be fixed. In library
> > we'd use definition identical to ARM. For kernel we'd use compat
> > syscall.
>
> I'm not sure I understand this part. What changes specifically do we need?
>

For kernel:

diff --git a/arch/arm64/include/asm/shmparam.h b/arch/arm64/include/asm/shmparam.h
index 4df608a..e368a55 100644
--- a/arch/arm64/include/asm/shmparam.h
+++ b/arch/arm64/include/asm/shmparam.h
@@ -21,7 +21,7 @@
* alignment value. Since we don't have aliasing D-caches, the rest of
* the time we can safely use PAGE_SIZE.
*/
-#define COMPAT_SHMLBA 0x4000
+#define COMPAT_SHMLBA (4 * PAGE_SIZE)

#include <asm-generic/shmparam.h>

diff --git a/arch/arm64/kernel/sys_ilp32.c b/arch/arm64/kernel/sys_ilp32.c
index c5bc712..877bedf 100644
--- a/arch/arm64/kernel/sys_ilp32.c
+++ b/arch/arm64/kernel/sys_ilp32.c
@@ -42,15 +42,12 @@ asmlinkage long sys_rt_sigreturn_wrapper(void);
#define compat_sys_pwrite64 sys_pwrite64
#define compat_sys_readahead sys_readahead
#define compat_sys_rt_sigaction sys_rt_sigaction
-#define compat_sys_shmat sys_shmat
#define compat_sys_sync_file_range sys_sync_file_range
#define compat_sys_truncate64 sys_truncate
#define compat_sys_sigaltstack sys_sigaltstack

For library - just create a header in ilp32 directory that defines
SHMLBA exactly as arm: __getpagesize () << 2

> It sounds like shmat is one of the cases we an override makes sense
> and we should use sys_shmat with PAGE_SIZE for aarch64 ilp32 mode.
>

[...]

> We normally assume that the page size on ARM is fixed to 4K, so
> there might be user space that just hardcodes 16K SHMLBA

It means that we should use compat_sys_shmat because hardcoded
userspace may have a chance to work for 4K pages. If we'll use
sys_shmat (and so 4K SHMLBA), we'll definitely make hardcoded
userspace broken. Non-hardcoded userspace will work anyway.

We can describe in documentation that 4k pages are prefferable.

Yury