[PATCH 2/3] careadlinkat: avoid ptrdiff_t overflow

[Paul Eggert <eggert@cs.ucla.edu>]

* lib/careadlinkat.c: Include idx.h, minmax.h.
(readlink_stk): Avoid ptrdiff_t overflow in object allocation.
Since this module uses arbitrary allocators (including
stdlib_allocator), it cannot assume GNU malloc semantics.
* modules/careadlinkat (Depends-on): Add idx, minmax.
---
 ChangeLog            |  7 +++++++
 lib/careadlinkat.c   | 28 +++++++++++-----------------
 modules/careadlinkat |  2 ++
 3 files changed, 20 insertions(+), 17 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 3aaee32bf..1e6cbd07f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2021-04-21  Paul Eggert  <eggert@cs.ucla.edu>
 
+	careadlinkat: avoid ptrdiff_t overflow
+	* lib/careadlinkat.c: Include idx.h, minmax.h.
+	(readlink_stk): Avoid ptrdiff_t overflow in object allocation.
+	Since this module uses arbitrary allocators (including
+	stdlib_allocator), it cannot assume GNU malloc semantics.
+	* modules/careadlinkat (Depends-on): Add idx, minmax.
+
 	execute-tests: pacify compiler
 	* tests/test-execute-main.c (main): Use 0x7DEADBEE rather than
 	0xDEADBEEF for nonces, to avoid provoking AIX XLC compiler warning
diff --git a/lib/careadlinkat.c b/lib/careadlinkat.c
index 18cfc114b..d833a0bce 100644
--- a/lib/careadlinkat.c
+++ b/lib/careadlinkat.c
@@ -22,6 +22,9 @@
 
 #include "careadlinkat.h"
 
+#include "idx.h"
+#include "minmax.h"
+
 #include <errno.h>
 #include <limits.h>
 #include <string.h>
@@ -65,11 +68,6 @@ readlink_stk (int fd, char const *filename,
               ssize_t (*preadlinkat) (int, char const *, char *, size_t),
               char stack_buf[STACK_BUF_SIZE])
 {
-  char *buf;
-  size_t buf_size;
-  size_t buf_size_max =
-    SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
-
   if (! alloc)
     alloc = &stdlib_allocator;
 
@@ -79,14 +77,14 @@ readlink_stk (int fd, char const *filename,
       buffer_size = STACK_BUF_SIZE;
     }
 
-  buf = buffer;
-  buf_size = buffer_size;
+  char *buf = buffer;
+  idx_t buf_size_max = MIN (IDX_MAX, MIN (SSIZE_MAX, SIZE_MAX));
+  idx_t buf_size = MIN (buffer_size, buf_size_max);
 
   while (buf)
     {
       /* Attempt to read the link into the current buffer.  */
-      ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
-      size_t link_size;
+      idx_t link_length = preadlinkat (fd, filename, buf, buf_size);
       if (link_length < 0)
         {
           if (buf != buffer)
@@ -98,7 +96,7 @@ readlink_stk (int fd, char const *filename,
           return NULL;
         }
 
-      link_size = link_length;
+      idx_t link_size = link_length;
 
       if (link_size < buf_size)
         {
@@ -127,17 +125,13 @@ readlink_stk (int fd, char const *filename,
       if (buf != buffer)
         alloc->free (buf);
 
-      if (buf_size < buf_size_max / 2)
-        buf_size = 2 * buf_size + 1;
-      else if (buf_size < buf_size_max)
-        buf_size = buf_size_max;
-      else if (buf_size_max < SIZE_MAX)
+      if (buf_size_max / 2 <= buf_size)
         {
           errno = ENAMETOOLONG;
           return NULL;
         }
-      else
-        break;
+
+      buf_size = 2 * buf_size + 1;
       buf = alloc->allocate (buf_size);
     }
 
diff --git a/modules/careadlinkat b/modules/careadlinkat
index 3f49aaecd..b3375a9b2 100644
--- a/modules/careadlinkat
+++ b/modules/careadlinkat
@@ -7,6 +7,8 @@ lib/careadlinkat.h
 
 Depends-on:
 allocator
+idx
+minmax
 ssize_t
 unistd
 
-- 
2.27.0