[newlib-cygwin/main] nano-mallocr: Prevent NULL pointer de-reference in free_list

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=c8397ae8171f00dcfb071130e6ea2b64aea17ded

commit c8397ae8171f00dcfb071130e6ea2b64aea17ded
Author:     Henrik Nilsson via Newlib <newlib@sourceware.org>
AuthorDate: Fri Feb 17 06:56:49 2023 +0100
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Mon Feb 27 10:54:26 2023 +0100

    nano-mallocr: Prevent NULL pointer de-reference in free_list
    
    The existing code checked if there was a chunk in free_list and
    that the tail was not the next chunk.
    
    The check if there is a chunk is not needed since it's already
    known but the case of a single chunk in free_list needs to be
    handled differently.

Diff:
---
 newlib/libc/stdlib/nano-mallocr.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/newlib/libc/stdlib/nano-mallocr.c b/newlib/libc/stdlib/nano-mallocr.c
index b2273ba60943..a2b50facc35e 100644
--- a/newlib/libc/stdlib/nano-mallocr.c
+++ b/newlib/libc/stdlib/nano-mallocr.c
@@ -333,14 +333,23 @@ void * nano_malloc(RARG malloc_size_t s)
                {
                    p->size += alloc_size;
 
-                   /* Remove chunk from free_list */
+                   /* Remove chunk from free_list. Since p != NULL there is
+                      at least one chunk */
                    r = free_list;
-                   while (r && p != r->next)
+                   if (r->next == NULL)
                    {
-                     r = r->next;
+                       /* There is only a single chunk, remove it */
+                       free_list = NULL;
+                   }
+                   else
+                   {
+                       /* Search for the chunk before the one to be removed */
+                       while (p != r->next)
+                       {
+                           r = r->next;
+                       }
+                       r->next = NULL;
                    }
-                   r->next = NULL;
-
                    r = p;
                }
                else