Commit 2667e069 authored by Alex Bennée's avatar Alex Bennée
Browse files

linux-user: don't use MAP_FIXED in pgd_find_hole_fallback 



Plain MAP_FIXED has the undesirable behaviour of splatting exiting
maps so we don't actually achieve what we want when looking for gaps.
We should be using MAP_FIXED_NOREPLACE. As this isn't always available
we need to potentially check the returned address to see if the kernel
gave us what we asked for.

Fixes: ad592e37 ("linux-user: provide fallback pgd_find_hole for bare chroots")
Signed-off-by: default avatarAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: default avatarRichard Henderson <richard.henderson@linaro.org>
Message-Id: <20200724064509.331-9-alex.bennee@linaro.org>
parent 163b3d1a

include/qemu/osdep.h

+3 −0
Original line number Diff line number Diff line
@@ -173,6 +173,9 @@ extern int daemon(int, int); 
#ifndef MAP_ANONYMOUS

#define MAP_ANONYMOUS MAP_ANON

#endif

#ifndef MAP_FIXED_NOREPLACE

#define MAP_FIXED_NOREPLACE 0

#endif

#ifndef ENOMEDIUM

#define ENOMEDIUM ENODEV

#endif

linux-user/elfload.c

+6 −4
Original line number Diff line number Diff line
@@ -2134,13 +2134,16 @@ static uintptr_t pgd_find_hole_fallback(uintptr_t guest_size, uintptr_t brk, 
            /* we have run out of space */

            return -1;

        } else {

            int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE | MAP_FIXED;

            int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE |

                MAP_FIXED_NOREPLACE;

            void * mmap_start = mmap((void *) align_start, guest_size,

                                     PROT_NONE, flags, -1, 0);

            if (mmap_start != MAP_FAILED) {

                munmap((void *) align_start, guest_size);

                if (MAP_FIXED_NOREPLACE || mmap_start == (void *) align_start) {

                    return (uintptr_t) mmap_start + offset;

                }

            }

            base += qemu_host_page_size;

        }

    }
@@ -2307,9 +2310,8 @@ static void pgb_reserved_va(const char *image_name, abi_ulong guest_loaddr, 
    /* Widen the "image" to the entire reserved address space. */

    pgb_static(image_name, 0, reserved_va, align);



#ifdef MAP_FIXED_NOREPLACE

    /* osdep.h defines this as 0 if it's missing */

    flags |= MAP_FIXED_NOREPLACE;

#endif



    /* Reserve the memory on the host. */

    assert(guest_base != 0);