[newlib-cygwin] libm/stdlib: don't read past source in nano_realloc

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=ce4044adeebfdc60714d3a35f67ba536edb55612

commit ce4044adeebfdc60714d3a35f67ba536edb55612
Author: Keith Packard via Newlib <newlib@sourceware.org>
Date:   Thu Aug 13 17:19:01 2020 -0700

    libm/stdlib: don't read past source in nano_realloc
    
    Save the computed block size and use it to avoid reading past
    the end of the source block.
    
    Signed-off-by: Keith Packard <keithp@keithp.com>

Diff:
---
 newlib/libc/stdlib/nano-mallocr.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/newlib/libc/stdlib/nano-mallocr.c b/newlib/libc/stdlib/nano-mallocr.c
index 9c59cff73..3970753a9 100644
--- a/newlib/libc/stdlib/nano-mallocr.c
+++ b/newlib/libc/stdlib/nano-mallocr.c
@@ -466,6 +466,7 @@ void * nano_realloc(RARG void * ptr, malloc_size_t size)
 {
     void * mem;
     chunk * p_to_realloc;
+    malloc_size_t old_size;
 
     if (ptr == NULL) return nano_malloc(RCALL size);
 
@@ -477,13 +478,14 @@ void * nano_realloc(RARG void * ptr, malloc_size_t size)
 
     /* TODO: There is chance to shrink the chunk if newly requested
      * size is much small */
-    if (nano_malloc_usable_size(RCALL ptr) >= size)
+    old_size = nano_malloc_usable_size(RCALL ptr);
+    if (old_size >= size)
       return ptr;
 
     mem = nano_malloc(RCALL size);
     if (mem != NULL)
     {
-        memcpy(mem, ptr, size);
+        memcpy(mem, ptr, old_size);
         nano_free(RCALL ptr);
     }
     return mem;