On 2020/9/1 2:15, Yury Norov wrote:

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On Mon, Aug 31, 2020 at 5:48 AM Xiongfeng Wang
[off-list ref] wrote:

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Hi Yury,

Hi Xiongfeng,

[restore CC list]

Haven't seen this before. What kernel / glibc / ltp do you use?

The kernel version is 4.19. I applied the ILP32 patches from
https://github.com/norov/linux.git. The glibc version is 2.28 and I applyed the
ILP32 patches.
The ltp testsuite is from https://github.com/linux-test-project/ltp. I build it
with '-mabi=ilp32'.

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We were testing the ILP32 feature and came accross a problem. Very apperaciate
it if you could give us some help !

We compile the LTP testsuite with '-mabi=ilp32' and run it on a machine with
kernel and glibc applied with ILP32 patches. But we failed on one testcase,
prctl04. It print the following error info.
'prctl04.c:199: FAIL: SECCOMP_MODE_STRICT doesn't permit read(2) write(2) and
_exit(2)'

The testcase is like below, syscall 'prctl' followed by a syscall 'write'.
prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);
SAFE_WRITE(1, fd, "a", 1);

When we execute syscall 'write', we receive a SIGKILL. It's not as expected.
We track the kernel and found out it is because we failed the syscall_whitelist
check in '__secure_computing_strict'. Because flag 'TIF_32BIT_AARCH64' is set,
we falls into the 'in_compat_syscall()' branch. We compare the parameter
'this_syscall' with return value of 'get_compat_model_syscalls()'
The syscall number of '__NR_write' for ilp32 application is 64, but it is 4 for
'model_syscalls_32' returned from 'get_compat_model_syscalls()'
So '__secure_computing_strict' retuned with 'do_exit(SIGKILL)'. We have a
modification like below, but I am not sure if it correct or not.

--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c

@@ -618,7 +618,7 @@ static void __secure_computing_strict(int this_syscall)
 {
        const int *syscall_whitelist = mode1_syscalls;
 #ifdef CONFIG_COMPAT
-       if (in_compat_syscall())
+       if (is_a32_compat_task())
                syscall_whitelist = get_compat_mode1_syscalls();

It calls the arch function from generic code. It may break build for
other arches.
This also looks dangerous because it treats ILP32 execution as non-compat.

The right approach would be implementing arch-specific
get_compat_mode1_syscalls()
in arch/arm64/include/asm/seccomp.h that returns an appropriate table.
Refer MIPS
code for this: arch/mips/include/asm/seccomp.h

Thanks for your advice. Thanks a lot.
I have written another version according to your advice.

--- a/arch/arm64/include/asm/seccomp.h
+++ b/arch/arm64/include/asm/seccomp.h

@@ -20,6 +20,36 @@
 #define __NR_seccomp_sigreturn_32      __NR_compat_rt_sigreturn
 #endif /* CONFIG_COMPAT */

+#ifdef CONFIG_COMPAT
+#ifndef __COMPAT_SYSCALL_NR
+
+static inline const int *get_compat_mode1_syscalls(void)
+{
+#ifdef CONFIG_AARCH32_EL0
+       static const int mode1_syscalls_a32[] = {
+               __NR_compat_read, __NR_compat_write,
+               __NR_compat_read, __NR_compat_sigreturn,