Re: [PATCH 06/21] powerpc: Avoid comparison of unsigned long >= 0 in __access_ok

[Christophe LEROY]

Le 26/02/2018 à 07:34, Christophe LEROY a écrit :


Le 25/02/2018 à 18:22, Mathieu Malaterre a écrit :
Rewrite check size - 1 <= Y as size < Y since `size` is unsigned value.
Fix warning (treated as error in W=1):

   CC      arch/powerpc/kernel/signal_32.o
In file included from ./include/linux/uaccess.h:14:0,
                  from ./include/asm-generic/termios-base.h:8,
                  from ./arch/powerpc/include/asm/termios.h:20,
                  from ./include/uapi/linux/termios.h:6,
                  from ./include/linux/tty.h:7,
                  from arch/powerpc/kernel/signal_32.c:36:
./include/asm-generic/termios-base.h: In function 
‘user_termio_to_kernel_termios’:
./arch/powerpc/include/asm/uaccess.h:52:35: error: comparison of 
unsigned expression >= 0 is always true [-Werror=type-limits]
    (((size) == 0) || (((size) - 1) <= ((segment).seg - (addr)))))
                                    ^
./arch/powerpc/include/asm/uaccess.h:58:3: note: in expansion of macro 
‘__access_ok’
    __access_ok((__force unsigned long)(addr), (size), get_fs()))
    ^~~~~~~~~~~
./arch/powerpc/include/asm/uaccess.h:262:6: note: in expansion of 
macro ‘access_ok’
   if (access_ok(VERIFY_READ, __gu_addr, (size)))   \
       ^~~~~~~~~
./arch/powerpc/include/asm/uaccess.h:80:2: note: in expansion of macro 
‘__get_user_check’
   __get_user_check((x), (ptr), sizeof(*(ptr)))
   ^~~~~~~~~~~~~~~~
./include/asm-generic/termios-base.h:36:6: note: in expansion of macro 
‘get_user’
   if (get_user(termios->c_line, &termio->c_line) < 0)
       ^~~~~~~~
[...]
cc1: all warnings being treated as errors

Signed-off-by: Mathieu Malaterre <ma...@debian.org>
---
  arch/powerpc/include/asm/uaccess.h | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/uaccess.h 
b/arch/powerpc/include/asm/uaccess.h
index 51bfeb8777f0..fadc406bd39d 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -49,7 +49,7 @@
  #define __access_ok(addr, size, segment)    \
      (((addr) <= (segment).seg) &&        \
-     (((size) == 0) || (((size) - 1) <= ((segment).seg - (addr)))))
+     (((size) == 0) || ((size) < ((segment).seg - (addr)))))

IIUC, ((2 - 1) <= 1) is the same as (2 < 1) ?????


Note that I already try to submit a fix for this warning 3 years ago 
(https://patchwork.ozlabs.org/patch/418075/) and it was rejected with 
the following comment:

Again, I don't think Linux enables this warning.  What did you do to
produce this?  In any case, it's a bad warning that doesn't take macros
into account, and the answer is not to make the code less clear by hiding
the fact that zero is a special case.

Christophe



Christophe

  #endif