Re: AC_FUNC_MMAP lacks <unistd.h> include?

[Matt Turner]

On Wed, May 10, 2023 at 8:26 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
>
> On 5/9/23 18:40, Sam James wrote:
> > It looks like this is still the case in master as well.
>
> Thanks for reporting that. Simply including unistd.h isn't enough in
> general, as POSIX no longer requires getpagesize to be declared there.
> So I instead installed the attached patch, which changes the test so
> that it doesn't use getpagesize. This is a conservative-enough approach
> that it should be OK to squeeze it in before the next Autoconf release.

Thanks. Unfortunately that doesn't resolve the issue and might even
make it worse.

I'm configuring libgtop with clang-15 on an amd64 system using:

> CC="clang" CXX="clang" CFLAGS="-std=c99 -Werror=implicit-function-declaration 
> -Werror=implicit-int -Werror=incompatible-function-pointer-types" ./autogen.sh

With an unpatched autoconf, AC_FUNC_MMAP produces this output in config.log:

> configure:17004: checking for working mmap
> configure:17156: clang -o conftest -std=c99 
> -Werror=implicit-function-declaration -Werror=implicit-int 
> -Werror=incompatible-function-pointer-types   conftest.c  >&5
> conftest.c:137:14: error: call to undeclared function 'getpagesize'; ISO C99 
> and later do not support implicit function declarations 
> [-Werror,-Wimplicit-function-declaration]
>   pagesize = getpagesize ();
>              ^
> 1 error generated.
> configure:17156: $? = 1
> configure: program exited with status 1

(My patch was useless because the conftest.c file was already
including unistd.h. I assume some #define is necessary to expose the
prototype for getpagesize)

And with your patch:

> configure:17004: checking for working mmap
> configure:17121: clang -o conftest -std=c99 
> -Werror=implicit-function-declaration -Werror=implicit-int 
> -Werror=incompatible-function-pointer-types   conftest.c  >&5
> configure:17121: $? = 0
> configure:17121: ./conftest
> ./configure: line 2069: 909713 Bus error               (core dumped) 
> ./conftest$ac_exeext
> configure:17121: $? = 135
> configure: program exited with status 135

The SIGBUS comes from

Program received signal SIGBUS, Bus error.
0x000055555555536d in main () at conftest.c:130
130        if (*(data2 + i))
(gdb) l
125        return 5;
126      data2 = (char *) mmap (0, pagesize, PROT_READ | PROT_WRITE,
MAP_SHARED, fd2, 0L);
127      if (data2 == MAP_FAILED)
128        return 6;
129      for (i = 0; i < pagesize; ++i)
130        if (*(data2 + i))
131          return 7;
132      close (fd2);
133      if (munmap (data2, pagesize))
134        return 8;
(gdb) p i
$1 = 4096


Changing pagesize s/8192/4096/ in the program makes it work.

I suspect the comment above this code indicates what the problem is:

  /* Next, check that the tail of a page is zero-filled.  File must have
     non-zero length, otherwise we risk SIGBUS for entire page.  */

We're mmap'ing a 1-byte file, and since the pagesize on amd64 is 4096
bytes, only the first 4096 byte page (of two) is mapped. Accessing the
second page triggers the SIGBUS.