[Qemu-devel] [PATCH,APPLIED] Usermode exec-stack fix

[Paul Brook]

When loading a shared library that requires an executable stack,
glibc uses the mprotext PROT_GROWSDOWN flag to achieve this.
We don't support PROT_GROWSDOWN.
Add a special case to handle changing the stack permissions in this way.

Signed-off-by: Paul Brook <address@hidden>
---
 linux-user/elfload.c  |    1 +
 linux-user/flatload.c |    1 +
 linux-user/qemu.h     |    1 +
 linux-user/syscall.c  |   11 +++++++++++
 4 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 2d920f2..accb44d 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -1018,6 +1018,7 @@ static abi_ulong setup_arg_pages(abi_ulong p, struct 
linux_binprm *bprm,
     /* we reserve one extra page at the top of the stack as guard */
     target_mprotect(error + size, qemu_host_page_size, PROT_NONE);
 
+    info->stack_limit = error;
     stack_base = error + size - MAX_ARG_PAGES*TARGET_PAGE_SIZE;
     p += stack_base;
 
diff --git a/linux-user/flatload.c b/linux-user/flatload.c
index 914de1f..8ad130a 100644
--- a/linux-user/flatload.c
+++ b/linux-user/flatload.c
@@ -802,6 +802,7 @@ int load_flt_binary(struct linux_binprm * bprm, struct 
target_pt_regs * regs,
     info->end_data = libinfo[0].end_data;
     info->start_brk = libinfo[0].start_brk;
     info->start_stack = sp;
+    info->stack_limit = libinfo[0].start_brk;
     info->entry = start_addr;
     info->code_offset = info->start_code;
     info->data_offset = info->start_data - libinfo[0].text_len;
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index dab3597..1878d5a 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -42,6 +42,7 @@ struct image_info {
         abi_ulong       mmap;
         abi_ulong       rss;
         abi_ulong       start_stack;
+        abi_ulong       stack_limit;
         abi_ulong       entry;
         abi_ulong       code_offset;
         abi_ulong       data_offset;
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index e94f1ee..0ebe7e1 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -5400,6 +5400,17 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
arg1,
         ret = get_errno(target_munmap(arg1, arg2));
         break;
     case TARGET_NR_mprotect:
+        {
+            TaskState *ts = ((CPUState *)cpu_env)->opaque;
+            /* Special hack to detect libc making the stack executable.  */
+            if ((arg3 & PROT_GROWSDOWN)
+                && arg1 >= ts->info->stack_limit
+                && arg1 <= ts->info->start_stack) {
+                arg3 &= ~PROT_GROWSDOWN;
+                arg2 = arg2 + arg1 - ts->info->stack_limit;
+                arg1 = ts->info->stack_limit;
+            }
+        }
         ret = get_errno(target_mprotect(arg1, arg2, arg3));
         break;
 #ifdef TARGET_NR_mremap
-- 
1.7.1