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From: | Thomas Huth |
Subject: | Re: [PATCH] linux-user/syscall: Silence warning from the undefined behavior sanitizer |
Date: | Fri, 12 Feb 2021 08:45:39 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 |
On 11/02/2021 22.28, Laurent Vivier wrote:
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. Signed-off-by: Thomas Huth <thuth@redhat.com> --- linux-user/syscall.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-) diff --git a/linux-user/syscall.c b/linux-user/syscall.c index 34760779c8..50de535ade 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -10491,20 +10491,22 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1, return -TARGET_EFAULT; ret = get_errno(sys_getdents64(arg1, dirp, count)); if (!is_error(ret)) { - struct linux_dirent64 *de; + char *de; int len = ret; int reclen; - de = dirp; + de = (char *)dirp; + #define de64(x) offsetof(struct linux_dirent64, x)Do we really need the cast to the "(char *)"? can't we use "&de->XXX" with the accessors? We don't access the memory, only read the address, the compiler should be happy.
That's what I thought and tried first, too. Unfortunately, it did not help to fix the issue, I had to take the detour via the char*
I guess the compiler also checks the alignment of the pointer when it gets assigned to the next record below ("de = ...").
Thomas
while (len > 0) { - reclen = de->d_reclen; + reclen = lduw_he_p(de + de64(d_reclen));to avoid human error, it would be better to let the compiler take the good accessor: ldn_he_p(&de->d_reclen, sizeof(de->d_reclen))if (reclen > len) break; - de->d_reclen = tswap16(reclen); - tswap64s((uint64_t *)&de->d_ino); - tswap64s((uint64_t *)&de->d_off); - de = (struct linux_dirent64 *)((char *)de + reclen); + stw_p(de + de64(d_reclen), reclen); + stq_p(de + de64(d_ino), ldq_he_p(de + de64(d_ino))); + stq_p(de + de64(d_off), ldq_he_p(de + de64(d_off)));and stwn_he_p() here too.+ de += reclen; len -= reclen; } + #undef de64 } unlock_user(dirp, arg2, ret); }Thank you Thomas for your help. Laurent
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