[PATCH v5 2/3] linux-user: Optimize memory layout for static and dynamic executables

[Helge Deller <deller@gmx.de>]

Reorganize the guest memory layout to get as much memory as possible for
heap for the guest application.

This patch optimizes the memory layout by loading pie executables
into lower memory and shared libs into higher memory (at
TASK_UNMAPPED_BASE). This leaves a bigger memory area usable for heap
space which will be located directly after the executable.
Up to now, pie executable and shared libs were loaded directly behind
each other in the area at TASK_UNMAPPED_BASE, which leaves very little
space for heap.

I tested this patchset with chroots of alpha, arm, armel, arm64, hppa, m68k,
mips64el, mipsel, powerpc, ppc64, ppc64el, s390x, sh4 and sparc64 on a x86-64
host, and with a static armhf binary (which fails to run without this patch).

This patch temporarily breaks the Thread Sanitizer (TSan) application
which expects specific boundary definitions for memory mappings on
different platforms [1], see commit aab613fb9597 ("linux-user: Update
TASK_UNMAPPED_BASE for aarch64") for aarch64. The follow-up patch fixes it
again.

[1] https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/tsan/rtl/tsan_platform.h

Signed-off-by: Helge Deller <deller@gmx.de>
---
 linux-user/elfload.c | 55 +++++++++++++-------------------------------
 linux-user/mmap.c    |  8 ++++---
 2 files changed, 21 insertions(+), 42 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 861ec07abc..47a118e430 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -3023,6 +3023,7 @@ static void load_elf_image(const char *image_name, int image_fd,
     abi_ulong load_addr, load_bias, loaddr, hiaddr, error;
     int i, retval, prot_exec;
     Error *err = NULL;
+    bool is_main_executable;

     /* First of all, some simple consistency checks */
     if (!elf_check_ident(ehdr)) {
@@ -3106,28 +3107,8 @@ static void load_elf_image(const char *image_name, int image_fd,
         }
     }

-    if (pinterp_name != NULL) {
-        /*
-         * This is the main executable.
-         *
-         * Reserve extra space for brk.
-         * We hold on to this space while placing the interpreter
-         * and the stack, lest they be placed immediately after
-         * the data segment and block allocation from the brk.
-         *
-         * 16MB is chosen as "large enough" without being so large as
-         * to allow the result to not fit with a 32-bit guest on a
-         * 32-bit host. However some 64 bit guests (e.g. s390x)
-         * attempt to place their heap further ahead and currently
-         * nothing stops them smashing into QEMUs address space.
-         */
-#if TARGET_LONG_BITS == 64
-        info->reserve_brk = 32 * MiB;
-#else
-        info->reserve_brk = 16 * MiB;
-#endif
-        hiaddr += info->reserve_brk;
-
+    is_main_executable = (pinterp_name != NULL);
+    if (is_main_executable) {
         if (ehdr->e_type == ET_EXEC) {
             /*
              * Make sure that the low address does not conflict with
@@ -3136,7 +3117,7 @@ static void load_elf_image(const char *image_name, int image_fd,
             probe_guest_base(image_name, loaddr, hiaddr);
         } else {
             /*
-             * The binary is dynamic, but we still need to
+             * The binary is dynamic (pie-executabe), but we still need to
              * select guest_base.  In this case we pass a size.
              */
             probe_guest_base(image_name, 0, hiaddr - loaddr);
@@ -3159,7 +3140,7 @@ static void load_elf_image(const char *image_name, int image_fd,
      */
     load_addr = target_mmap(loaddr, (size_t)hiaddr - loaddr + 1, PROT_NONE,
                             MAP_PRIVATE | MAP_ANON | MAP_NORESERVE |
-                            (ehdr->e_type == ET_EXEC ? MAP_FIXED : 0),
+                            (is_main_executable ? MAP_FIXED : 0),
                             -1, 0);
     if (load_addr == -1) {
         goto exit_mmap;
@@ -3194,7 +3175,8 @@ static void load_elf_image(const char *image_name, int image_fd,
     info->end_code = 0;
     info->start_data = -1;
     info->end_data = 0;
-    info->brk = 0;
+    /* possible start for brk is behind all sections of this ELF file. */
+    info->brk = TARGET_PAGE_ALIGN(hiaddr);
     info->elf_flags = ehdr->e_flags;

     prot_exec = PROT_EXEC;
@@ -3288,9 +3270,6 @@ static void load_elf_image(const char *image_name, int image_fd,
                     info->end_data = vaddr_ef;
                 }
             }
-            if (vaddr_em > info->brk) {
-                info->brk = vaddr_em;
-            }
 #ifdef TARGET_MIPS
         } else if (eppnt->p_type == PT_MIPS_ABIFLAGS) {
             Mips_elf_abiflags_v0 abiflags;
@@ -3618,6 +3597,15 @@ int load_elf_binary(struct linux_binprm *bprm, struct image_info *info)

     if (elf_interpreter) {
         load_elf_interp(elf_interpreter, &interp_info, bprm->buf);
+        /*
+	 * Use brk address of interpreter if it was loaded above the
+	 * executable and leaves less than 16 MB for heap.
+	 * This happens e.g. with static binaries on armhf.
+         */
+        if (interp_info.brk > info->brk &&
+            interp_info.load_bias - info->brk < 16 * MiB)  {
+            info->brk = interp_info.brk;
+        }

         /* If the program interpreter is one of these two, then assume
            an iBCS2 image.  Otherwise assume a native linux image.  */
@@ -3672,17 +3660,6 @@ int load_elf_binary(struct linux_binprm *bprm, struct image_info *info)
     bprm->core_dump = &elf_core_dump;
 #endif

-    /*
-     * If we reserved extra space for brk, release it now.
-     * The implementation of do_brk in syscalls.c expects to be able
-     * to mmap pages in this space.
-     */
-    if (info->reserve_brk) {
-        abi_ulong start_brk = HOST_PAGE_ALIGN(info->brk);
-        abi_ulong end_brk = HOST_PAGE_ALIGN(info->brk + info->reserve_brk);
-        target_munmap(start_brk, end_brk - start_brk);
-    }
-
     return 0;
 }

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index a5dfb56545..848d2fd4bb 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -299,14 +299,16 @@ static bool mmap_frag(abi_ulong real_start, abi_ulong start, abi_ulong last,
 #ifdef TARGET_AARCH64
 # define TASK_UNMAPPED_BASE  0x5500000000
 #else
-# define TASK_UNMAPPED_BASE  (1ul << 38)
+# define TASK_UNMAPPED_BASE  0x4000000000
 #endif
-#else
+#elif HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 32
 #ifdef TARGET_HPPA
 # define TASK_UNMAPPED_BASE  0xfa000000
 #else
-# define TASK_UNMAPPED_BASE  0x40000000
+# define TASK_UNMAPPED_BASE  0xe0000000
 #endif
+#else /* HOST_LONG_BITS == 32 && TARGET_ABI_BITS == 32 */
+# define TASK_UNMAPPED_BASE  0x40000000
 #endif
 abi_ulong mmap_next_start = TASK_UNMAPPED_BASE;

--
2.41.0