Re: Alignment issue with sha1 code from gnulib

[Jim Meyering]

David Shaw <address@hidden> wrote:
> Peter Palfrader reported a bug against the sha1 code in paperkey, but
> that code actually comes from gnulib, so I'm referring it to you.
>
> The issue comes up (as noted in the comment) if resbuf is not 32-bit
> aligned.  Rather than requiring all programs that use the gnulib sha1
> code to align their result buffer, Peter's patch seems to make more
> sense in that it "just works" on any platform.
>
> David
>
> ----- Forwarded message from Peter Palfrader <address@hidden> -----
>
> This seems to fix the issue:
>
> -sha1_read_ctx (const struct sha1_ctx *ctx, void *resbuf)
> +sha1_read_ctx (const struct sha1_ctx *ctx, char *resbuf)
>  {
> -  ((uint32_t *) resbuf)[0] = SWAP (ctx->A);
> -  ((uint32_t *) resbuf)[1] = SWAP (ctx->B);
> -  ((uint32_t *) resbuf)[2] = SWAP (ctx->C);
> -  ((uint32_t *) resbuf)[3] = SWAP (ctx->D);
> -  ((uint32_t *) resbuf)[4] = SWAP (ctx->E);
> +  set_uint32(&resbuf[0*4], SWAP (ctx->A));
> +  set_uint32(&resbuf[1*4], SWAP (ctx->B));
> +  set_uint32(&resbuf[2*4], SWAP (ctx->C));
> +  set_uint32(&resbuf[3*4], SWAP (ctx->D));
> +  set_uint32(&resbuf[4*4], SWAP (ctx->E));

Thanks for the suggestion.  It looks like a good one.
However, I don't want to change the type of the resbuf parameter.
Here's the change I'm considering:

diff --git a/lib/sha1.c b/lib/sha1.c
index 035f898..d9e99d8 100644
--- a/lib/sha1.c
+++ b/lib/sha1.c
@@ -67,28 +67,32 @@ sha1_init_ctx (struct sha1_ctx *ctx)
   ctx->buflen = 0;
 }

-/* Put result from CTX in first 20 bytes following RESBUF.  The result
-   must be in little endian byte order.
+/* Copy the 4 byte value from v into the memory location pointed to by *cp,
+   If your architecture allows unaligned access this is equivalent to
+   * (uint32_t *) cp = v  */
+void
+set_uint32 (char *cp, uint32_t v)
+{
+  memcpy (cp, &v, 4);
+}

-   IMPORTANT: On some systems it is required that RESBUF is correctly
-   aligned for a 32-bit value.  */
+/* Put result from CTX in first 20 bytes following RESBUF.  The result
+   must be in little endian byte order.  */
 void *
 sha1_read_ctx (const struct sha1_ctx *ctx, void *resbuf)
 {
-  ((uint32_t *) resbuf)[0] = SWAP (ctx->A);
-  ((uint32_t *) resbuf)[1] = SWAP (ctx->B);
-  ((uint32_t *) resbuf)[2] = SWAP (ctx->C);
-  ((uint32_t *) resbuf)[3] = SWAP (ctx->D);
-  ((uint32_t *) resbuf)[4] = SWAP (ctx->E);
+  char *r = resbuf;
+  set_uint32 (r + 0*4, SWAP (ctx->A));
+  set_uint32 (r + 1*4, SWAP (ctx->B));
+  set_uint32 (r + 2*4, SWAP (ctx->C));
+  set_uint32 (r + 3*4, SWAP (ctx->D));
+  set_uint32 (r + 4*4, SWAP (ctx->E));

   return resbuf;
 }

 /* Process the remaining bytes in the internal buffer and the usual
-   prolog according to the standard and write the result to RESBUF.
-
-   IMPORTANT: On some systems it is required that RESBUF is correctly
-   aligned for a 32-bit value.  */
+   prolog according to the standard and write the result to RESBUF.  */
 void *
 sha1_finish_ctx (struct sha1_ctx *ctx, void *resbuf)
 {
diff --git a/lib/sha1.h b/lib/sha1.h
index 9545f0b..9e55974 100644
--- a/lib/sha1.h
+++ b/lib/sha1.h
@@ -1,6 +1,7 @@
 /* Declarations of functions and data types used for SHA1 sum
    library functions.
-   Copyright (C) 2000, 2001, 2003, 2005, 2006 Free Software Foundation, Inc.
+   Copyright (C) 2000, 2001, 2003, 2005, 2006, 2008
+   Free Software Foundation, Inc.

    This program is free software; you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by the
@@ -57,19 +58,13 @@ extern void sha1_process_bytes (const void *buffer, size_t 
len,
 /* Process the remaining bytes in the buffer and put result from CTX
    in first 20 bytes following RESBUF.  The result is always in little
    endian byte order, so that a byte-wise output yields to the wanted
-   ASCII representation of the message digest.
-
-   IMPORTANT: On some systems it is required that RESBUF be correctly
-   aligned for a 32 bits value.  */
+   ASCII representation of the message digest.  */
 extern void *sha1_finish_ctx (struct sha1_ctx *ctx, void *resbuf);


 /* Put result from CTX in first 20 bytes following RESBUF.  The result is
    always in little endian byte order, so that a byte-wise output yields
-   to the wanted ASCII representation of the message digest.
-
-   IMPORTANT: On some systems it is required that RESBUF is correctly
-   aligned for a 32 bits value.  */
+   to the wanted ASCII representation of the message digest.  */
 extern void *sha1_read_ctx (const struct sha1_ctx *ctx, void *resbuf);