Re: [PATCH 17/22] Implement do_obreak function

[Richard Henderson]

On 8/19/23 02:48, Karim Taha wrote:
> From: Stacey Son <sson@FreeBSD.org>
>
> Co-authored-by: Mikaël Urankar <mikael.urankar@gmail.com>
>
> Signed-off-by: Mikaël Urankar <mikael.urankar@gmail.com>
> Signed-off-by: Stacey Son <sson@FreeBSD.org>
> Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
> ---
>   bsd-user/bsd-mem.h            | 85 +++++++++++++++++++++++++++++++++++
>   bsd-user/freebsd/os-syscall.c |  7 +++
>   2 files changed, 92 insertions(+)
>
> diff --git a/bsd-user/bsd-mem.h b/bsd-user/bsd-mem.h
> index edbccd3111..6f33148eb7 100644
> --- a/bsd-user/bsd-mem.h
> +++ b/bsd-user/bsd-mem.h
> @@ -167,4 +167,89 @@ static inline abi_long do_bsd_mincore(abi_ulong 
> target_addr, abi_ulong len,
>       return ret;
>   }
>   
> +#ifdef DO_DEBUG
> +#define DEBUGF_BRK(message, args...) \
> +    do { fprintf(stderr, (message), ## args); } while (0)
> +#else
> +#define DEBUGF_BRK(message, args...)
> +#endif

We are trying to get rid of all of this old-style debugging.
We are replacing them with tracepoints.

> +
> +/* do_brk() must return target values and target errnos. */
> +static inline abi_long do_obreak(abi_ulong new_brk)
> +{
> +    abi_long mapped_addr;
> +    int new_alloc_size;
> +
> +    DEBUGF_BRK("do_brk(" TARGET_ABI_FMT_lx ") -> ", new_brk);
> +
> +    if (!new_brk) {
> +        DEBUGF_BRK(TARGET_ABI_FMT_lx " (!new_brk)\n", bsd_target_brk);
> +        return bsd_target_brk;
> +    }
> +    if (new_brk < bsd_target_original_brk) {
> +        DEBUGF_BRK(TARGET_ABI_FMT_lx " (new_brk < bsd_target_original_brk)\n",
> +                   bsd_target_brk);
> +        return bsd_target_brk;
> +    }

These two cases may be combined, since original_brk will never be 0.

> +
> +    /*
> +     * If the new brk is less than the highest page reserved to the target heap
> +     * allocation, set it and we're almost done...
> +     */
> +    if (new_brk <= brk_page) {
> +        /*
> +         * Heap contents are initialized to zero, as for anonymous mapped 
> pages.
> +         */
> +        if (new_brk > bsd_target_brk) {
> +            memset(g2h_untagged(bsd_target_brk), 0, new_brk - bsd_target_brk);
> +        }
> +        bsd_target_brk = new_brk;
> +        DEBUGF_BRK(TARGET_ABI_FMT_lx " (new_brk <= brk_page)\n",
> +                   bsd_target_brk);
> +        return bsd_target_brk;
> +    }
> +
> +    /*
> +     * We need to allocate more memory after the brk... Note that we don't use
> +     * MAP_FIXED because that will map over the top of any existing mapping
> +     * (like the one with the host libc or qemu itself); instead we treat
> +     * "mapped but at wrong address" as a failure and unmap again.
> +     */
> +    new_alloc_size = HOST_PAGE_ALIGN(new_brk - brk_page);

We removed all HOST_PAGE_ALIGN from brk in linux-user this cycle.

> +    mapped_addr = get_errno(target_mmap(brk_page, new_alloc_size,
> +                                        PROT_READ | PROT_WRITE,
> +                                        MAP_ANON | MAP_PRIVATE, -1, 0));
> +
> +    if (mapped_addr == brk_page) {
> +        /*
> +         * Heap contents are initialized to zero, as for anonymous mapped 
> pages.
> +         * Technically the new pages are already initialized to zero since they
> +         * *are* anonymous mapped pages, however we have to take care with the
> +         * contents that come from the remaining part of the previous page: it
> +         * may contains garbage data due to a previous heap usage (grown then
> +         * shrunken).
> +         */
> +        memset(g2h_untagged(bsd_target_brk), 0, brk_page - bsd_target_brk);
> +
> +        bsd_target_brk = new_brk;
> +        brk_page = HOST_PAGE_ALIGN(bsd_target_brk);
> +        DEBUGF_BRK(TARGET_ABI_FMT_lx " (mapped_addr == brk_page)\n",
> +            bsd_target_brk);
> +        return bsd_target_brk;
> +    } else if (mapped_addr != -1) {
> +        /*
> +         * Mapped but at wrong address, meaning there wasn't actually enough
> +         * space for this brk.
> +         */
> +        target_munmap(mapped_addr, new_alloc_size);
> +        mapped_addr = -1;
> +        DEBUGF_BRK(TARGET_ABI_FMT_lx " (mapped_addr != -1)\n", bsd_target_brk);
> +    } else {
> +        DEBUGF_BRK(TARGET_ABI_FMT_lx " (otherwise)\n", bsd_target_brk);
> +    }

It may require additional work in target_mmap, but you probably want to use MAP_FIXED | 
MAP_EXCL here.  On the linux-user side we changed to use MAP_FIXED_NOREPLACE, which is 
approximately the same thing.


r~