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

Weidong Wang
2016-06-08T01:34:20Z

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