[Qemu-devel] [PATCH] linux-user: Handle images where lowest vaddr is not page aligned

[Qemu-devel] [PATCH] linux-user: Handle images where lowest vaddr is not page aligned

[Peter Maydell]

Fix a bug in the linux-user ELF loader code where it was not correctly
handling images where the lowest vaddr to be loaded was not page aligned.
The problem was that the code to probe for a suitable guest base address
was changing the 'loaddr' variable (by rounding it to a page boundary),
which meant that the load bias would then be incorrectly calculated
unless loaddr happened to already be page-aligned.

Binaries generated by gcc with the default linker script do start with
a loadable segment at a page-aligned vaddr, so were unaffected. This
bug was noticed with a binary created by the Google Go toolchain for ARM.

We fix the bug by refactoring the "probe for guest base" code out into
its own self-contained function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 linux-user/elfload.c |  130 ++++++++++++++++++++++++++++----------------------
 1 files changed, 73 insertions(+), 57 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 4c399f8..930f6d3 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -1269,6 +1269,78 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
     return sp;
 }
 
+static void probe_guest_base(const char *image_name,
+                             abi_ulong loaddr, abi_ulong hiaddr)
+{
+    /* Probe for a suitable guest base address, if the user has not set
+     * it explicitly, and set guest_base appropriately.
+     * In case of error we will print a suitable message and exit.
+     */
+#if defined(CONFIG_USE_GUEST_BASE)
+    const char *errmsg;
+    if (!have_guest_base && !reserved_va) {
+        unsigned long host_start, real_start, host_size;
+
+        /* Round addresses to page boundaries.  */
+        loaddr &= qemu_host_page_mask;
+        hiaddr = HOST_PAGE_ALIGN(hiaddr);
+
+        if (loaddr < mmap_min_addr) {
+            host_start = HOST_PAGE_ALIGN(mmap_min_addr);
+        } else {
+            host_start = loaddr;
+            if (host_start != loaddr) {
+                errmsg = "Address overflow loading ELF binary";
+                goto exit_errmsg;
+            }
+        }
+        host_size = hiaddr - loaddr;
+        while (1) {
+            /* Do not use mmap_find_vma here because that is limited to the
+               guest address space.  We are going to make the
+               guest address space fit whatever we're given.  */
+            real_start = (unsigned long)
+                mmap((void *)host_start, host_size, PROT_NONE,
+                     MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE, -1, 0);
+            if (real_start == (unsigned long)-1) {
+                goto exit_perror;
+            }
+            if (real_start == host_start) {
+                break;
+            }
+            /* That address didn't work.  Unmap and try a different one.
+               The address the host picked because is typically right at
+               the top of the host address space and leaves the guest with
+               no usable address space.  Resort to a linear search.  We
+               already compensated for mmap_min_addr, so this should not
+               happen often.  Probably means we got unlucky and host
+               address space randomization put a shared library somewhere
+               inconvenient.  */
+            munmap((void *)real_start, host_size);
+            host_start += qemu_host_page_size;
+            if (host_start == loaddr) {
+                /* Theoretically possible if host doesn't have any suitably
+                   aligned areas.  Normally the first mmap will fail.  */
+                errmsg = "Unable to find space for application";
+                goto exit_errmsg;
+            }
+        }
+        qemu_log("Relocating guest address space from 0x"
+                 TARGET_ABI_FMT_lx " to 0x%lx\n",
+                 loaddr, real_start);
+        guest_base = real_start - loaddr;
+    }
+    return;
+
+exit_perror:
+    errmsg = strerror(errno);
+exit_errmsg:
+    fprintf(stderr, "%s: %s\n", image_name, errmsg);
+    exit(-1);
+#endif
+}
+
+
 /* Load an ELF image into the address space.
 
    IMAGE_NAME is the filename of the image, to use in error messages.
@@ -1354,63 +1426,7 @@ static void load_elf_image(const char *image_name, int image_fd,
         /* This is the main executable.  Make sure that the low
            address does not conflict with MMAP_MIN_ADDR or the
            QEMU application itself.  */
-#if defined(CONFIG_USE_GUEST_BASE)
-        /*
-         * In case where user has not explicitly set the guest_base, we
-         * probe here that should we set it automatically.
-         */
-        if (!have_guest_base && !reserved_va) {
-            unsigned long host_start, real_start, host_size;
-
-            /* Round addresses to page boundaries.  */
-            loaddr &= qemu_host_page_mask;
-            hiaddr = HOST_PAGE_ALIGN(hiaddr);
-
-            if (loaddr < mmap_min_addr) {
-                host_start = HOST_PAGE_ALIGN(mmap_min_addr);
-            } else {
-                host_start = loaddr;
-                if (host_start != loaddr) {
-                    errmsg = "Address overflow loading ELF binary";
-                    goto exit_errmsg;
-                }
-            }
-            host_size = hiaddr - loaddr;
-            while (1) {
-                /* Do not use mmap_find_vma here because that is limited to the
-                   guest address space.  We are going to make the
-                   guest address space fit whatever we're given.  */
-                real_start = (unsigned long)
-                    mmap((void *)host_start, host_size, PROT_NONE,
-                         MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE, -1, 0);
-                if (real_start == (unsigned long)-1) {
-                    goto exit_perror;
-                }
-                if (real_start == host_start) {
-                    break;
-                }
-                /* That address didn't work.  Unmap and try a different one.
-                   The address the host picked because is typically right at
-                   the top of the host address space and leaves the guest with
-                   no usable address space.  Resort to a linear search.  We
-                   already compensated for mmap_min_addr, so this should not
-                   happen often.  Probably means we got unlucky and host
-                   address space randomization put a shared library somewhere
-                   inconvenient.  */
-                munmap((void *)real_start, host_size);
-                host_start += qemu_host_page_size;
-                if (host_start == loaddr) {
-                    /* Theoretically possible if host doesn't have any suitably
-                       aligned areas.  Normally the first mmap will fail.  */
-                    errmsg = "Unable to find space for application";
-                    goto exit_errmsg;
-                }
-            }
-            qemu_log("Relocating guest address space from 0x"
-                     TARGET_ABI_FMT_lx " to 0x%lx\n", loaddr, real_start);
-            guest_base = real_start - loaddr;
-        }
-#endif
+        probe_guest_base(image_name, loaddr, hiaddr);
     }
     load_bias = load_addr - loaddr;
 
-- 
1.7.1

Re: [Qemu-devel] [PATCH] linux-user: Handle images where lowest vaddr is not page aligned

[Russ Cox]

Thanks for tracking this down!

Re: [Qemu-devel] [PATCH] linux-user: Handle images where lowest vaddr is not page aligned

[Gustavo Niemeyer]

> Thanks for tracking this down!

Indeed, thanks a lot for the very fast action on this.

-- 
Gustavo Niemeyer
http://niemeyer.net
http://niemeyer.net/blog
http://niemeyer.net/twitter