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libc: Add overflow check in malloc() and realloc()

--- a/src/libc/stdlib/malloc.c

+++ b/src/libc/stdlib/malloc.c

@@ -136,6 +136,9 @@

 	Header *cur, *prev;

 	size_t nunits;

+        if (nbytes == 0 || nbytes > SIZE_MAX - sizeof(Header)-1)

+		return NULL;

+

 	/* 1 unit for header plus enough units to fit nbytes */

 	nunits = (nbytes+sizeof(Header)-1)/sizeof(Header) + 1;

--- a/src/libc/stdlib/realloc.c

+++ b/src/libc/stdlib/realloc.c

@@ -1,3 +1,4 @@

+#include <stdint.h>

 #include <stdlib.h>

 #include <string.h>

@@ -10,7 +11,7 @@

 	Header *oh, *prev, *next, *new;

 	size_t nunits, avail, onbytes, n;

-	if (nbytes == 0)

+	if (nbytes == 0 || nbytes > SIZE_MAX - sizeof(Header)-1)

 		return NULL;

 	if (!ptr)