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

From: Zhangjian (Bamvor) <hidden>
Date: 2016-06-08 02:17:55
Also in: lkml

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

quoted hunk

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 <redacted>
---
  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

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