On 12/02/2021 01.35, Laurent Vivier wrote:
Le 11/02/2021 à 22:28, Laurent Vivier a écrit :
Le 11/02/2021 à 14:29, Thomas Huth a écrit :
When compiling QEMU with -fsanitize=undefined, there is a warning when
running "make check-tcg":
TEST linux-test on m68k
../linux-user/syscall.c:10499:34: runtime error: member access within
misaligned address 0x00008006df3c for type 'struct linux_dirent64',
which requires 8 byte alignment
0x00008006df3c: note: pointer points here
00 00 00 00 68 03 28 00 00 00 00 00 5b 96 3e e4 61 4b 05 26 18 00 04 2e
00 00 00 00 da 3f 18 00
^
It's likely not an issue in reality, since I assume that on hosts where
the alignment really matters (like sparc64), the Linux kernel likely
adds the right padding. Anyway, let's use the stw_p() / stq_p() accessor
helpers here to silence the warning and thus to allow to compile the code
with -fsanitize=undefined, too.
Wait... if the alignment differs between m68k and the host, I guess the size of
the structure differs?
No, as far as I understand, the size and layout of the structure are the
same on all architecture. The problem is that the "dirp =
lock_user(VERIFY_WRITE, arg2, count, 0)" already ends up in a pointer that
is only aligned to a 4-byte boundary. Since the m68k code is only restricted
to a 4-byte alignment, arg2 is only aligned to 4 bytes. But if the host
needs 8-byte alignment for the struct, we've certainly lost here.
Having said that, I think my patch is still wrong. It silences the ubsan
warnings, but it won't fix the problem when the code is e.g. running on a
sparc64 host. There we likely need to make sure that the buffer on the host
is already aligned to an 8-byte boundary when doing the sys_getdents64()
call to the host kernel.
So I guess we need a bounce buffer here anyway?
Thomas
