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From:   "Amanieu d'Antras" <amanieu@gmail.com>
Date:   Mon, 6 Jan 2020 19:03:32 +0100
Message-ID: <CA+y5pbSBYLvZ46nJP0pSYZnRohtPxHitOHPEaLXq23-QrPKk2g@mail.gmail.com>
Subject: Re: [PATCH 2/7] arm64: Implement copy_thread_tls
To:     Will Deacon <will@kernel.org>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org,
        Christian Brauner <christian@brauner.io>,
        "# 3.4.x" <stable@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Mon, Jan 6, 2020 at 6:39 PM Will Deacon <will@kernel.org> wrote:
> I also ran the native and compat selftests but, unfortunately, they all
> pass even without this patch. Do you reckon it would be possible to update
> them to check the tls pointer?

Here's the program I used for testing on arm64. I considered adding it
to the selftests but there is no portable way of reading the TLS
register on all architectures.

#include <sys/syscall.h>
#include <unistd.h>
#include <stdio.h>
#include <stdint.h>

#define __NR_clone3 435
struct clone_args {
    uint64_t flags;
    uint64_t pidfd;
    uint64_t child_tid;
    uint64_t parent_tid;
    uint64_t exit_signal;
    uint64_t stack;
    uint64_t stack_size;
    uint64_t tls;
};

#define USE_CLONE3

int main() {
    printf("Before fork: tp = %p\n", __builtin_thread_pointer());
#ifdef USE_CLONE3
    struct clone_args args = {
        .flags = CLONE_SETTLS,
        .tls = (uint64_t)__builtin_thread_pointer(),
    };
    int ret = syscall(__NR_clone3, &args, sizeof(args));
#else
    int ret = syscall(__NR_clone, CLONE_SETTLS, 0, 0,
__builtin_thread_pointer(), 0);
#endif
    printf("Fork returned %d, tp = %p\n", ret, __builtin_thread_pointer());
}