Re: [PATCH v2] Support madvise(MADV_DONTDUMP) when creating core dumps for qemu-user

[Philippe Mathieu-Daudé]

Hi Jon,

On 2/5/25 08:31, WorksButNotTested wrote:
> When running applications which make large (sparsely populated) address ranges
> (e.g. when using address sanitizer with LibAFL) the inability to exclude these
> regions from any core dump can result in very large files which fill the disk.
> A coredump is obvously very useful for performing a post-mortem when fuzzing.
>
> Whilst the man pages state that madvise provides only a hint (and hence can be
> ignored), this patch adds support to handle MADV_DONTDUMP and set a
> corresponding flag in the page flags, thus allowing QEMU to exclude these
> regions from the core file.
>
> Signed-off-by: WorksButNotTested 
> <62701594+WorksButNotTested@users.noreply.github.com>

In v1 you use Jon Wilson <jonwilson030981@googlemail.com>,
is this other address the one you really want to use here?

> ---
>   include/exec/page-protection.h | 6 ++++++
>   linux-user/elfload.c           | 4 ++++
>   linux-user/mmap.c              | 5 +++++
>   3 files changed, 15 insertions(+)
>
> diff --git a/include/exec/page-protection.h b/include/exec/page-protection.h
> index c43231af8b..f8826d917e 100644
> --- a/include/exec/page-protection.h
> +++ b/include/exec/page-protection.h
> @@ -38,4 +38,10 @@
>    */
>   #define PAGE_PASSTHROUGH 0x0800
>   
> +/*
> + * For linux-user, indicates that the page should not be included in a core
> + * dump.
> + */
> +#define PAGE_DONTDUMP   0x1000
> +
>   #endif
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index fbfdec2f17..41c46da055 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -4067,6 +4067,10 @@ static size_t vma_dump_size(target_ulong start, 
> target_ulong end,
>           return 0;
>       }
>   
> +    if (flags & PAGE_DONTDUMP) {
> +        return 0;
> +    }
> +
>       /*
>        * Usually we don't dump executable pages as they contain
>        * non-writable code that debugger can read directly from
> diff --git a/linux-user/mmap.c b/linux-user/mmap.c
> index f88a80c31e..62fc8ac921 100644
> --- a/linux-user/mmap.c
> +++ b/linux-user/mmap.c
> @@ -1247,6 +1247,11 @@ abi_long target_madvise(abi_ulong start, abi_ulong 
> len_in, int advice)
>        */
>       mmap_lock();
>       switch (advice) {
> +    case MADV_DONTDUMP:
> +        if (len > 0) {
> +            page_set_flags(start, start + len - 1, PAGE_DONTDUMP);
> +        }
> +        break;
>       case MADV_WIPEONFORK:
>       case MADV_KEEPONFORK:
>           ret = -EINVAL;