[PATCH v3] hurd: readv: Get rid of alloca

[PATCH v3] hurd: readv: Get rid of alloca

[Joe Simmons-Talbott]

Replace alloca with a scratch_buffer to avoid potential stack overflows.

Checked on i686-gnu and x86_64-linux-gnu
---
Changes since v2:
  * Add back unneccessarily removed blank line.
  * Add space between scratch_buffer_free and (.

 sysdeps/posix/readv.c | 28 ++++++++++++----------------
 1 file changed, 12 insertions(+), 16 deletions(-)

diff --git a/sysdeps/posix/readv.c b/sysdeps/posix/readv.c
index 91208e9894..6a85319eab 100644
--- a/sysdeps/posix/readv.c
+++ b/sysdeps/posix/readv.c
@@ -19,6 +19,7 @@
 #include <unistd.h>
 #include <string.h>
 #include <limits.h>
+#include <scratch_buffer.h>
 #include <stdbool.h>
 #include <sys/param.h>
 #include <sys/uio.h>
@@ -26,9 +27,9 @@
 
 
 static void
-ifree (char **ptrp)
+ifree (struct scratch_buffer *sbuf)
 {
-  free (*ptrp);
+  scratch_buffer_free (sbuf);
 }
 
 /* Read data from file descriptor FD, and put the result in the
@@ -52,20 +53,15 @@ __readv (int fd, const struct iovec *vector, int count)
       bytes += vector[i].iov_len;
     }
 
-  /* Allocate a temporary buffer to hold the data.  We should normally
-     use alloca since it's faster and does not require synchronization
-     with other threads.  But we cannot if the amount of memory
-     required is too large.  */
-  char *buffer;
-  char *malloced_buffer __attribute__ ((__cleanup__ (ifree))) = NULL;
-  if (__libc_use_alloca (bytes))
-    buffer = (char *) __alloca (bytes);
-  else
-    {
-      malloced_buffer = buffer = (char *) malloc (bytes);
-      if (buffer == NULL)
-	return -1;
-    }
+  /* Allocate a temporary buffer to hold the data.  Use a scratch_buffer
+     since it's faster for small buffer sizes but can handle larger
+     allocations as well.  */
+     
+  struct scratch_buffer __attribute__ ((__cleanup__ (ifree))) buf;
+  scratch_buffer_init (&buf);
+  if (!scratch_buffer_set_array_size (&buf, 1, bytes))
+    return -1;
+  char *buffer = buf.data;
 
   /* Read the data.  */
   ssize_t bytes_read = __read (fd, buffer, bytes);
-- 
2.39.2

Re: [PATCH v3] hurd: readv: Get rid of alloca

[Samuel Thibault]

Fixed extra spaces and pushed, thanks!

Joe Simmons-Talbott via Libc-alpha, le lun. 19 juin 2023 10:43:34 -0400, a ecrit:
> Replace alloca with a scratch_buffer to avoid potential stack overflows.
> 
> Checked on i686-gnu and x86_64-linux-gnu
> ---
> Changes since v2:
>   * Add back unneccessarily removed blank line.
>   * Add space between scratch_buffer_free and (.
> 
>  sysdeps/posix/readv.c | 28 ++++++++++++----------------
>  1 file changed, 12 insertions(+), 16 deletions(-)
> 
> diff --git a/sysdeps/posix/readv.c b/sysdeps/posix/readv.c
> index 91208e9894..6a85319eab 100644
> --- a/sysdeps/posix/readv.c
> +++ b/sysdeps/posix/readv.c
> @@ -19,6 +19,7 @@
>  #include <unistd.h>
>  #include <string.h>
>  #include <limits.h>
> +#include <scratch_buffer.h>
>  #include <stdbool.h>
>  #include <sys/param.h>
>  #include <sys/uio.h>
> @@ -26,9 +27,9 @@
>  
>  
>  static void
> -ifree (char **ptrp)
> +ifree (struct scratch_buffer *sbuf)
>  {
> -  free (*ptrp);
> +  scratch_buffer_free (sbuf);
>  }
>  
>  /* Read data from file descriptor FD, and put the result in the
> @@ -52,20 +53,15 @@ __readv (int fd, const struct iovec *vector, int count)
>        bytes += vector[i].iov_len;
>      }
>  
> -  /* Allocate a temporary buffer to hold the data.  We should normally
> -     use alloca since it's faster and does not require synchronization
> -     with other threads.  But we cannot if the amount of memory
> -     required is too large.  */
> -  char *buffer;
> -  char *malloced_buffer __attribute__ ((__cleanup__ (ifree))) = NULL;
> -  if (__libc_use_alloca (bytes))
> -    buffer = (char *) __alloca (bytes);
> -  else
> -    {
> -      malloced_buffer = buffer = (char *) malloc (bytes);
> -      if (buffer == NULL)
> -	return -1;
> -    }
> +  /* Allocate a temporary buffer to hold the data.  Use a scratch_buffer
> +     since it's faster for small buffer sizes but can handle larger
> +     allocations as well.  */
> +     
> +  struct scratch_buffer __attribute__ ((__cleanup__ (ifree))) buf;
> +  scratch_buffer_init (&buf);
> +  if (!scratch_buffer_set_array_size (&buf, 1, bytes))
> +    return -1;
> +  char *buffer = buf.data;
>  
>    /* Read the data.  */
>    ssize_t bytes_read = __read (fd, buffer, bytes);
> -- 
> 2.39.2
> 

-- 
Samuel
---
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