[PATCH] Removed the unsafe ngx_memcmp() wrapper for memcmp(3)

Maxim Dounin mdounin at mdounin.ru
Sat Nov 5 02:39:26 UTC 2022


On Fri, Nov 04, 2022 at 04:24:14PM +0100, Alejandro Colomar wrote:

> The casts are unnecessary, since memcmp(3)'s arguments are
> 'const void *', which allows implicit conversion from any pointer type.
> It might have been necessary in the times of K&R C, where 'void *'
> didn't exist yet, up until the early 2000s, because some old systems
> still had limited or no support for ISO C89.  Those systems passed away
> a long time ago, and current systems, even the oldest living ones, have
> support for ISO C89.
> The changes, apart from the removal of the macro itself, were scripted:
> $ find src/ -type f \
>   | grep '\.[ch]$' \
>   | xargs sed -i 's/ngx_memcmp/memcmp/';
> The cast in this case is (almost) innocuous, because it only hides
> warnings for conversions from integer to pointer such as:
>     nxt_memcmp(n, "foo", 3);  // no warnings
>     memcmp(n, "foo", 3);      // warning: integer to pointer conversion
> which is a difficult bug to write, since it's too obvious.  Such code
> will probably be caught in a code review, but there's always a small
> risk.  Since there's no reason to keep the small risk around, when we
> can just avoid it by removing the cast.
> In general, it's better to avoid a cast if possible, since casts will
> disable many compiler warnings regarding type safety.

Thanks for the patch.

The ngx_memcpy() macro was introduced in 107:b5be4b0448d3 
(nginx-0.0.1-2003-07-01-19:00:03 import) as an alias to memcmp(), 
with the following explanation why it's just an alias:

+/* msvc and icc compile memcmp() to inline loop */
+#define ngx_memcmp                memcmp

This was along with the first use of memcmp in nginx, so clearly 
the "ngx_" prefix is not an accident.

Indeed, the prefix is used in multiple places to provide optimized 
or instrumented functions, even if normally they map to generally 
available functions such as memcmp().  For example, ngx_memcpy() 
usually maps to memcpy(), but instrumented to detect large copies 
if NGX_MEMCPY_LIMIT is defined.  Even if the particular function 
is not instrumented, the prefix is important to make it possible 
to change things without huge modifications of the code, like the 
one in your patch.  Further, the prefix is also important for code 
consistency.  And, last but not least, it makes it possible to add 
casts if needed on some platforms (or remove them if they are 
considered wrong).

In particular, casts to (const char *) for ngx_memcmp() were added 
in 2007 by this commit (nginx 0.6.18):

changeset:   1648:89a47f19b9ec
user:        Igor Sysoev <igor at sysoev.ru>
date:        Fri Nov 23 17:00:11 2007 +0000
summary:     update ngx_memcmp()

Unfortunately, this commit contains no details about the 
particular system which needed the cast, though it looks like 
there were some at least at that time.

Similar casts are used by almost all nginx string macro 
definitions, since most C library functions accept "char *", while 
nginx uses "u_char *" for strings, and this results in useless 
warnings.  While it might be seen as something better to avoid, 
the only alternative would be to provide wrapper functions, which 
might not be a good idea for performance and readability reasons.  
On the other hand, macro definitions with casts are easily 
readable, have no impact on performance, and provide limited to 
no effect on the code quality as long as proper coding, testing 
and review process is implemented.

Summing the above, certainly removing the "ngx_" prefix is a bad 
idea.  Removing casts from the particular macro might be 
considered, though in general it looks like a quest to prove these 
are not needed (or no longer needed) given the wide range of 
platforms nginx supports.

Hope this helps.


Maxim Dounin

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