Re: [PATCH v2] linux-user: Add emulation for MADV_WIPEONFORK and MADV_KEEPONFORK in madvise()

[Laurent Vivier <laurent@vivier.eu>]

Le 13/12/2022 à 18:03, Helge Deller a écrit :
> Both parameters have a different value on the parisc platform, so first
> translate the target value into a host value for usage in the native
> madvise() syscall.
> 
> Those parameters are often used by security sensitive applications (e.g.
> tor browser, boringssl, ...) which expect the call to return a proper
> return code on failure, so return -EINVAL if qemu fails to forward the
> syscall to the host OS.
> 
> While touching this code, enhance the comments about MADV_DONTNEED.
> 
> Tested with testcase of tor browser when running hppa-linux guest on
> x86-64 host.
> 
> Signed-off-by: Helge Deller <deller@gmx.de>
> 
> ---
> v2: based on feedback from Ilya Leoshkevich <iii@linux.ibm.com>
> - rename can_passthrough_madv_dontneed() to can_passthrough_madvise()
> - rephrase the comment about MADV_DONTNEED
> 
> diff --git a/linux-user/mmap.c b/linux-user/mmap.c
> index 10f5079331..28135c9e6a 100644
> --- a/linux-user/mmap.c
> +++ b/linux-user/mmap.c
> @@ -857,7 +857,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size,
>       return new_addr;
>   }
> 
> -static bool can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end)
> +static bool can_passthrough_madvise(abi_ulong start, abi_ulong end)
>   {
>       ulong addr;
> 
> @@ -901,23 +901,53 @@ abi_long target_madvise(abi_ulong start, abi_ulong len_in, int advice)
>           return -TARGET_EINVAL;
>       }
> 
> +    /* Translate for some architectures which have different MADV_xxx values */
> +    switch (advice) {
> +    case TARGET_MADV_DONTNEED:      /* alpha */
> +        advice = MADV_DONTNEED;
> +        break;
> +    case TARGET_MADV_WIPEONFORK:    /* parisc */
> +        advice = MADV_WIPEONFORK;
> +        break;
> +    case TARGET_MADV_KEEPONFORK:    /* parisc */
> +        advice = MADV_KEEPONFORK;
> +        break;
> +    /* we do not care about the other MADV_xxx values yet */
> +    }
> +
>       /*
> -     * A straight passthrough may not be safe because qemu sometimes turns
> -     * private file-backed mappings into anonymous mappings.
> +     * Most advice values are hints, so ignoring and returning success is ok.
> +     *
> +     * However, some advice values such as MADV_DONTNEED, MADV_WIPEONFORK and
> +     * MADV_KEEPONFORK are not hints and need to be emulated.
>        *
> -     * This is a hint, so ignoring and returning success is ok.
> +     * A straight passthrough for those may not be safe because qemu sometimes
> +     * turns private file-backed mappings into anonymous mappings.
> +     * can_passthrough_madvise() helps to check if a passthrough is possible by
> +     * comparing mappings that are known to have the same semantics in the host
> +     * and the guest. In this case passthrough is safe.
>        *
> -     * This breaks MADV_DONTNEED, completely implementing which is quite
> -     * complicated. However, there is one low-hanging fruit: mappings that are
> -     * known to have the same semantics in the host and the guest. In this case
> -     * passthrough is safe, so do it.
> +     * We pass through MADV_WIPEONFORK and MADV_KEEPONFORK if possible and
> +     * return failure if not.
> +     *
> +     * MADV_DONTNEED is passed through as well, if possible.
> +     * If passthrough isn't possible, we nevertheless (wrongly!) return
> +     * success, which is broken but some userspace programs fail to work
> +     * otherwise. Completely implementing such emulation is quite complicated
> +     * though.
>        */
>       mmap_lock();
> -    if (advice == TARGET_MADV_DONTNEED &&
> -        can_passthrough_madv_dontneed(start, end)) {
> -        ret = get_errno(madvise(g2h_untagged(start), len, MADV_DONTNEED));
> -        if (ret == 0) {
> -            page_reset_target_data(start, start + len);
> +    switch (advice) {
> +    case MADV_WIPEONFORK:
> +    case MADV_KEEPONFORK:
> +        ret = -EINVAL;
> +        /* fall through */
> +    case MADV_DONTNEED:
> +        if (can_passthrough_madvise(start, end)) {
> +            ret = get_errno(madvise(g2h_untagged(start), len, advice));
> +            if ((advice == MADV_DONTNEED) && (ret == 0)) {
> +                page_reset_target_data(start, start + len);
> +            }
>           }
>       }
>       mmap_unlock();
> 

Applied to my linux-user-for-8.0 branch.

Thanks,
Laurent