[RFC PATCH] sys_read: add a compat_sys_read for 64bit system

[Weidong Wang <wangweidong1@huawei.com>]

Test 32 progress and 64 progress on the 64bit system with
this progress:

int main(int argc, char **argv)
{
        int fd = 0;
        int i, ret = 0;
        char buf[512];
        unsigned long count = -1;

        fd = open("/tmp", O_RDONLY);
        if (fd < -1) {
                printf("Pls check the directory is exist?\n");
                return -1;
        }
        errno = 0;
        ret = read(fd, NULL, count);
        printf("Ret is %d errno %d\n", ret, errno);
        close(fd);

        return 0;
}

we get the different errno. The 64 progress we get errno is -14 while
the 32 progress is -21.

The reason is that, the user progress would use a 32bit count, while
the sys_read size_t in kernel is 64bit.  When the uesrspace count is
-1(0xffffffff), it goes to the sys_read, it would be change to a positive
number.

So I think we should add a compat_sys_read for the read syscall. I test it
on x86 or arm64 platform. The patch works well.

As well this patch may do work for the 'tile' 64 system.
I think it may enter the same result on mips/parisc/powerpc/sparc.
The s390 do the compat_sys_s390_read for the compat sys_read.

Signed-off-by: Weidong Wang <wangweidong1@huawei.com>
---
 arch/x86/entry/syscalls/syscall_32.tbl | 2 +-
 fs/read_write.c                        | 8 ++++++++
 include/linux/compat.h                 | 2 ++
 include/uapi/asm-generic/unistd.h      | 2 +-
 4 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 4cddd17..ebc24e3 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -9,7 +9,7 @@
 0	i386	restart_syscall		sys_restart_syscall
 1	i386	exit			sys_exit
 2	i386	fork			sys_fork			sys_fork
-3	i386	read			sys_read
+3	i386	read			sys_read			compat_sys_read
 4	i386	write			sys_write
 5	i386	open			sys_open			compat_sys_open
 6	i386	close			sys_close
diff --git a/fs/read_write.c b/fs/read_write.c
index 933b53a..d244848 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -613,6 +613,14 @@ SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
 	return ret;
 }

+#ifdef CONFIG_COMPAT
+COMPAT_SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf,
+		compat_size_t, count)
+{
+        return sys_read(fd, buf, (compat_ssize_t)count);
+}
+#endif
+
 SYSCALL_DEFINE4(pread64, unsigned int, fd, char __user *, buf,
 			size_t, count, loff_t, pos)
 {
diff --git a/include/linux/compat.h b/include/linux/compat.h
index f964ef7..d88ccad 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -332,6 +332,8 @@ asmlinkage long compat_sys_keyctl(u32 option,
 			      u32 arg2, u32 arg3, u32 arg4, u32 arg5);
 asmlinkage long compat_sys_ustat(unsigned dev, struct compat_ustat __user *u32);

+asmlinkage ssize_t compat_sys_read(unsigned int fd,
+		char __user * buf, compat_size_t count);
 asmlinkage ssize_t compat_sys_readv(compat_ulong_t fd,
 		const struct compat_iovec __user *vec, compat_ulong_t vlen);
 asmlinkage ssize_t compat_sys_writev(compat_ulong_t fd,
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index a26415b..745818a 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -201,7 +201,7 @@ __SC_COMP(__NR_getdents64, sys_getdents64, compat_sys_getdents64)
 #define __NR3264_lseek 62
 __SC_3264(__NR3264_lseek, sys_llseek, sys_lseek)
 #define __NR_read 63
-__SYSCALL(__NR_read, sys_read)
+__SC_COMP(__NR_read, sys_read, compat_sys_read)
 #define __NR_write 64
 __SYSCALL(__NR_write, sys_write)
 #define __NR_readv 65
-- 
2.7.0