brk: COMPAT_BRK: fix detection of randomized brk

5520e89 ("brk: fix min_brk lower bound computation for COMPAT_BRK")
tried to get the whole logic of brk randomization for legacy
(libc5-based) applications finally right.

It turns out that the way to detect whether brk has actually been
randomized in the end or not introduced by that patch still doesn't work
for those binaries, as reported by Geert:

: /sbin/init from my old m68k ramdisk exists prematurely.
:
: Before the patch:
:
: | brk(0x80005c8e)                         = 0x80006000
:
: After the patch:
:
: | brk(0x80005c8e)                         = 0x80005c8e
:
: Old libc5 considers brk() to have failed if the return value is not
: identical to the requested value.

I don't like it, but currently see no better option than a bit flag in
task_struct to catch the CONFIG_COMPAT_BRK && randomize_va_space == 2
case.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 9 insertions, 2 deletions

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index f34078d702d3..303983fabfd6 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -941,9 +941,13 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
         current->mm->start_stack = bprm->p;
 
 #ifdef arch_randomize_brk
-        if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1))
+        if ((current->flags & PF_RANDOMIZE) && (randomize_va_space > 1)) {
                 current->mm->brk = current->mm->start_brk =
                         arch_randomize_brk(current->mm);
+#ifdef CONFIG_COMPAT_BRK
+                current->brk_randomized = 1;
+#endif
+        }
 #endif
 
         if (current->personality & MMAP_PAGE_ZERO) {
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4ec2c027e92c..18d63cea2848 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1254,6 +1254,9 @@ struct task_struct {
 #endif
 
         struct mm_struct *mm, *active_mm;
+#ifdef CONFIG_COMPAT_BRK
+        unsigned brk_randomized:1;
+#endif
 #if defined(SPLIT_RSS_COUNTING)
         struct task_rss_stat    rss_stat;
 #endif
diff --git a/mm/mmap.c b/mm/mmap.c
index 8c05e5b43b69..e27e0cf0de03 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -259,7 +259,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
          * randomize_va_space to 2, which will still cause mm->start_brk
          * to be arbitrarily shifted
          */
-        if (mm->start_brk > PAGE_ALIGN(mm->end_data))
+        if (current->brk_randomized)
                 min_brk = mm->start_brk;
         else
                 min_brk = mm->end_data;