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

Zhangjian (Bamvor)
2016-06-08T11:47:36Z

Hi, Peter

On 2016/6/8 15:33, H. Peter Anvin wrote:
> On June 7, 2016 7:14:41 PM PDT, "Zhangjian (Bamvor)" <bamvor.zhangjian@huawei.com> wrote:
>> Hi,
>>
>> On 2016/6/8 9:33, Weidong Wang wrote:
>>> 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 weidong said, we tested on x86, x86_64, aarch64 ilp32, aarch64 lp64.
>> We do not familiar with other architecture, cc linux-api, hope could
>> get more
>> input.
>>
>> Regards
>>
>> Bamvor
>>>
>>> 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
>>>
>
> Does this cause any actual problems? Also, it seems extremely unlikely read() would be the only system call so affected.
It do not cause any actual problems. And write may be affected too.
Without this patch, the errno is different when 32 application migrate
from 32bit kernel to 64bit kernel.

Regards

Bamvor

>