FreeBSD compatibility... #110

oschonrock · 2024-12-08T16:00:11Z

Is FreeBSD supported?

The following code snippet leads to a segfault on FreeBSD 14, but works fine on Linux (Ubuntu 24.04 or on Windows/mingw) :

#include "mio/mmap.hpp"                                                                                                                                                                                                                                       
#include <iostream>                                                                                                                                                                                                                                           
#include <system_error>                                                                                                                                                                                                                                       
                                                                                                                                                                                                                                                              
int main() {                                                                                                                                                                                                                                                  
  std::error_code err;                                                                                                                                                                                                                                        
  mio::mmap_sink mmap;                                                                                                                                                                                                                                        
  mmap.map("test/tmp/sharded_filter8.bin", err);                                                                                                                                                                                                              
  if (err) std::cerr << err.message() << "\n";                                                                                                                                                                                                                
  std::string tag(15, '\0');                                                                                                                                                                                                                                  
  memcpy(tag.data(), mmap.data(), 15);                                                                                                                                                                                                                        
  std::cout << "'" << tag << "'\n";                                                                                                                                                                                                                                                                                                                                                                                                                                              
}

precise error (obtained with valgrind): 
==3608== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==3608==  Bad permissions for mapped region at address 0x4841000
==3608==    at 0x203092: main (app/test_mio.cpp:11)     // line 11 is the memcpy

I am using mio::access_mode::write because in the original code it is writing and and then reading. If I change it to mmap_source it works fine.

I definitely have read and write permissions to the file, and an earlier instance of mmap_sink could write to the file.
I single stepped through the code, and is correctly opening the file with O_RDWR and passing PROT_WRITE to the ::mmap call.

If I change mmap.ipp:L208 to always pass PROT_READ, it works fine.

The text was updated successfully, but these errors were encountered:

oschonrock · 2024-12-08T17:30:43Z

The linux (ubuntu 24.04) man page for mmap says:

       The prot argument describes the desired memory protection of the mapping (and must not conflict with the open mode of the file).  It is either PROT_NONE or the bitwise OR of one or more of the following flags:

       PROT_EXEC  Pages may be executed.
       PROT_READ  Pages may be read.
       PROT_WRITE Pages may be written.
       PROT_NONE  Pages may not be accessed.

the FreeBSD14 man page for mmap says:

     The protections (region accessibility) are specified in the prot argument                                                                                                                                                                                
     by or'ing the following values:                                                                                                                                                                                                                          
                                                                                                                                                                                                                                                              
     PROT_NONE   Pages may not be accessed.                                                                                                                                                                                                                   
     PROT_READ   Pages may be read.                                                                                                                                                                                                                           
     PROT_WRITE  Pages may be written.                                                                                                                                                                                                                        
     PROT_EXEC   Pages may be executed.

So these are very similar. However, they both seem to suggest that if you want read/write access then you need to "or together" the PROT_READ and PROT_WRITE flags.

It seems that FreeBSD actually stricly requires PROT_READ if you want to read, and PROT_WRITE alone is not sufficient, and this seems a very reasonable interpretation of the POSIX docs by both systems.

so I tried changing the relevant mio code as follows:

--- ext/mio/include/mio/detail/mmap.ipp.orig	2024-12-08 17:27:01.903206000 +0000
+++ ext/mio/include/mio/detail/mmap.ipp	2024-12-08 17:27:07.038059000 +0000
@@ -205,7 +205,7 @@
     char* mapping_start = static_cast<char*>(::mmap(
             0, // Don't give hint as to where to map.
             length_to_map,
-            mode == access_mode::read ? PROT_READ : PROT_READ,
+            mode == access_mode::read ? PROT_READ : PROT_READ | PROT_WRITE,
             MAP_SHARED,
             file_handle,
             aligned_offset));

This fixes the issue for me on FreeBSD and still works fine on Linux. Windows is not affected as it is in the other #ifdef branch.

I think this change would make the mio library more faithfully adhere to its documented intent:

/**                                                                                                                                                                                                                                                           
 * This is the basis for all read-write mmap objects and should be preferred over                                                                                                                                                                             
 * directly using `basic_mmap`.                                                                                                                                                                                                                               
 */                                                                                                                                                                                                                                                           
template<typename ByteT>                                                                                                                                                                                                                                      
using basic_mmap_sink = basic_mmap<access_mode::write, ByteT>;

on a wider range of POSIX platforms, including FreeBSD.

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FreeBSD compatibility... #110

FreeBSD compatibility... #110

oschonrock commented Dec 8, 2024 •

edited

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oschonrock commented Dec 8, 2024 •

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FreeBSD compatibility... #110

FreeBSD compatibility... #110

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oschonrock commented Dec 8, 2024 •

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oschonrock commented Dec 8, 2024 •

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