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[v3,08/16] linux-user: don't use MAP_FIXED in pgd_find_hole_fallback

Message ID 20200724064509.331-9-alex.bennee@linaro.org
State Superseded
Headers show
Series candidate fixes for 5.1-rc1 (testing, semihosting, OOM tcg, x86 fpu) | expand

Commit Message

Alex Bennée July 24, 2020, 6:45 a.m. UTC 
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: ad592e37dfc ("linux-user: provide fallback pgd_find_hole for bare chroots")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Message-Id: <20200722062902.24509-9-alex.bennee@linaro.org>
---
 include/qemu/osdep.h |  3 +++
 linux-user/elfload.c | 10 ++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

-- 
2.20.1
diff mbox series

Patch

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 0b1298b3c91..20872e793e4 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -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
diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 7e7f642332d..fe9dfe795dd 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -2134,12 +2134,15 @@  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);
-                return (uintptr_t) mmap_start + offset;
+                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);