Subject: Re: [PATCH 1/3] hexdump: Implement macro for converting large buffers

From: Simon Horman <[email-masked]>
Date: Sat, 15 Feb 2025 16:36:12 +0000

+ David Laight

On Fri, Feb 14, 2025 at 10:24:34AM -0600, Nick Child wrote:
> Define for_each_line_in_hex_dump which loops over a buffer and calls
> hex_dump_to_buffer for each segment in the buffer. This allows the
> caller to decide what to do with the resulting string and is not
> limited by a specific printing format like print_hex_dump.
> 
> Signed-off-by: Nick Child <[email-masked]>
> ---
>  include/linux/printk.h | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 4217a9f412b2..559d4bfe0645 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -755,6 +755,27 @@ enum {
>  extern int hex_dump_to_buffer(const void *buf, size_t len, int rowsize,
>  			      int groupsize, char *linebuf, size_t linebuflen,
>  			      bool ascii);
> +/**
> + * for_each_line_in_hex_dump - iterate over buffer, converting into hex ASCII
> + * @i: offset in @buff
> + * @rowsize: number of bytes to print per line; must be 16 or 32
> + * @linebuf: where to put the converted data
> + * @linebuflen: total size of @linebuf, including space for terminating NUL
> + *		IOW >= (@rowsize * 2) + ((@rowsize - 1 / @groupsize)) + 1
> + * @groupsize: number of bytes to print at a time (1, 2, 4, 8; default = 1)
> + * @buf: data blob to dump
> + * @len: number of bytes in the @buf
> + */
> + #define for_each_line_in_hex_dump(i, rowsize, linebuf, linebuflen, groupsize, \
> +				   buf, len) \
> +	for ((i) = 0;							\
> +	     (i) < (len) &&						\
> +	     hex_dump_to_buffer((unsigned char *)(buf) + (i),		\
> +				min((len) - (i), rowsize),		\
> +				(rowsize), (groupsize), (linebuf),	\
> +				(linebuflen), false);			\
> +	     (i) += (rowsize) == 16 || (rowsize) == 32 ? (rowsize) : 16	\
> +	    )
>  #ifdef CONFIG_PRINTK
>  extern void print_hex_dump(const char *level, const char *prefix_str,
>  			   int prefix_type, int rowsize, int groupsize,

Hi Nick,

When compiling with gcc 7.5.0 (old, but still supported AFAIK) on x86_64
with patch 2/3 (and 1/3) applied I see this:

  CC      lib/hexdump.o
In file included from <command-line>:0:0:
lib/hexdump.c: In function 'print_hex_dump':
././include/linux/compiler_types.h:542:38: error: call to '__compiletime_assert_11' declared with attribute error: min((len) - (i), rowsize) signedness error
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
                                      ^
././include/linux/compiler_types.h:523:4: note: in definition of macro '__compiletime_assert'
    prefix ## suffix();    \
    ^~~~~~
././include/linux/compiler_types.h:542:2: note: in expansion of macro '_compiletime_assert'
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/minmax.h:93:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
  BUILD_BUG_ON_MSG(!__types_ok(ux, uy),  \
  ^~~~~~~~~~~~~~~~
./include/linux/minmax.h:98:2: note: in expansion of macro '__careful_cmp_once'
  __careful_cmp_once(op, x, y, __UNIQUE_ID(x_), __UNIQUE_ID(y_))
  ^~~~~~~~~~~~~~~~~~
./include/linux/minmax.h:105:19: note: in expansion of macro '__careful_cmp'
 #define min(x, y) __careful_cmp(min, x, y)
                   ^~~~~~~~~~~~~
./include/linux/printk.h:774:5: note: in expansion of macro 'min'
     min((len) - (i), rowsize),  \
     ^~~
lib/hexdump.c:272:2: note: in expansion of macro 'for_each_line_in_hex_dump'
  for_each_line_in_hex_dump(i, rowsize, linebuf, sizeof(linebuf),
  ^~~~~~~~~~~~~~~~~~~~~~~~~

Highlighting the min line in the macro for context, it looks like this:

	min((len) - (i), rowsize)

And in this case the types involved are:

	size_t len
	int i
	int rowsize

This is not a proposal, but I made a quick hack changing they type of rowsize
to size_t and the problem goes away. So I guess it is the type missmatch
between the two arguments to min that needs to be resolved somehow.

FWIIW, you should be able to reproduce this problem fairly easily using
the toolchain here:
https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.5.0/