[Qemu-devel] [PATCH] linux-user: Fix the computation of the requested heap size

["Cédric VINCENT" <cedric.vincent@st.com>]

Hi Peter & Martin,

On 2011-04-21 14:04:40, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> I've had a report from Martin Mohring that this patch breaks some
> programs which previously worked in linux-user mode (one wonders if
> they were overwriting some chunk of memory that they happened not
> to be using for anything important...)

Peter, actually your patch exposes two old bugs more explicitly (since
the error code is now correctly reported).  To complete your patchset,
just consider this contribution as "PATCH 4/3" ;)

Martin, I suppose you have encountered a problem with the memory
allocator used in Bash and Emacs.  I reduced the problem to the
test-case embedded in the commit message below.

Also, I wonder how this whole new implementation of the target brk
could help ScratchBox2? [1]

Regards,
Cédric

[1] http://lists.gnu.org/archive/html/qemu-devel/2011-04/msg01817.html

PS: feel free to ask me for a separated patch if you don't [like to]
    use "git am --scissors"

8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----8<----

There were two remaining bugs in the previous implementation of
do_brk():

    1. the value of "new_alloc_size" was one page too large when the
       requested brk was aligned on a host page boundary.

    2. no new pages should be (re-)allocated when the requested brk is
       in the range of the pages that were already allocated
       previsouly (for the same purpose).  Technically these pages are
       never unmapped in the current implementation.

The problem/fix can be reproduced/validated with the following test
case:

    #include <unistd.h>       /* syscall(2),      */
    #include <sys/syscall.h>  /* SYS_brk,         */
    #include <stdio.h>        /* puts(3),         */
    #include <stdlib.h>       /* exit(3), EXIT_*, */

    int main()
    {
        int current_brk = 0;
        int new_brk;
        int failure = 0;

        void test(int increment) {
            static int test_number = 0;
            test_number++;

            new_brk = syscall(SYS_brk, current_brk + increment);
            if (new_brk == current_brk) {
                printf("test %d fails\n", test_number);
                failure++;
            }

            current_brk = new_brk;
        }

        /* Initialization.  */
        test(0);

        /* Does QEMU overlap host pages?  */
        test(HOST_PAGE_SIZE);
        test(HOST_PAGE_SIZE);

        /* Does QEMU allocate the same host page twice?  */
        test(-HOST_PAGE_SIZE);
        test(HOST_PAGE_SIZE);

        if (!failure) {
            printf("success\n");
            exit(EXIT_SUCCESS);
        }
        else {
            exit(EXIT_FAILURE);
        }
    }

Signed-off-by: Cédric VINCENT <cedric.vincent@st.com>
Reviewed-by: Christophe Guillon <christophe.guillon@st.com>
Cc: Riku Voipio <riku.voipio@iki.fi>
---
 linux-user/syscall.c |   11 ++++++-----
 1 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index b975730..be27f53 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -709,16 +709,17 @@ char *target_strerror(int err)
 
 static abi_ulong target_brk;
 static abi_ulong target_original_brk;
+static abi_ulong brk_page;
 
 void target_set_brk(abi_ulong new_brk)
 {
     target_original_brk = target_brk = HOST_PAGE_ALIGN(new_brk);
+    brk_page = HOST_PAGE_ALIGN(target_brk);
 }
 
 /* do_brk() must return target values and target errnos. */
 abi_long do_brk(abi_ulong new_brk)
 {
-    abi_ulong brk_page;
     abi_long mapped_addr;
     int	new_alloc_size;
 
@@ -727,9 +728,8 @@ abi_long do_brk(abi_ulong new_brk)
     if (new_brk < target_original_brk)
         return target_brk;
 
-    brk_page = HOST_PAGE_ALIGN(target_brk);
-
-    /* If the new brk is less than this, set it and we're done... */
+    /* If the new brk is less than the highest page reserved to the
+     * target heap allocation, set it and we're done... */
     if (new_brk < brk_page) {
 	target_brk = new_brk;
     	return target_brk;
@@ -741,13 +741,14 @@ abi_long do_brk(abi_ulong new_brk)
      * 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 + 1);
+    new_alloc_size = HOST_PAGE_ALIGN(new_brk - brk_page);
     mapped_addr = get_errno(target_mmap(brk_page, new_alloc_size,
                                         PROT_READ|PROT_WRITE,
                                         MAP_ANON|MAP_PRIVATE, 0, 0));
 
     if (mapped_addr == brk_page) {
         target_brk = new_brk;
+        brk_page = HOST_PAGE_ALIGN(target_brk);
         return target_brk;
     } else if (mapped_addr != -1) {
         /* Mapped but at wrong address, meaning there wasn't actually
-- 
1.7.5.1