[Gambas-user] [Fwd: Gambas3 Pointers example] signal #6
Demosthenes Koptsis
demosthenesk at ...626...
Mon Jan 10 00:54:46 CET 2011
so i manage to make it work...
i saw an example that said to use PATH_MAX+1
https://buildsecurityin.us-cert.gov/bsi/articles/knowledge/coding/760-BSI.html
so i did
Example 1) ----------------------
' Gambas module file
'char *getcwd(char *buf, size_t size);
Extern getcwd(buf As Pointer, size As Integer) As Pointer In "libc:6"
Public Sub Main()
Dim pFunc As Pointer
Dim pBuf As Pointer
Dim sWorkingDirectory As String
'getcwd
pFunc = Alloc(4097)
pBuf = Alloc(4097)
pFunc = getcwd(pBuf, 4097)
sWorkingDirectory = Str@(pFunc)
Print sWorkingDirectory
Free(pFunc)
' Free(pBuf)
Print sWorkingDirectory
End
----------------------
a) note in 1st example that
Print sWorkingDirectory
Free(pFunc)
Print sWorkingDirectory
first print gives the correct output
/home/user/Documents/Development/Gambas3/Pointers7
and the second after Free(pFunc) gives this
�s&^@^@^@^@^@^@^@^@^@ocuments/Development/Gambas3/Pointers7
b) if i use Free(pBuf)
i get signal #6
and
*** glibc detected *** Pointers7: double free or corruption (!prev):
0x087c94c0 ***
Example 2 --------------------
' Gambas module file
'char *get_current_dir_name(void);
Extern get_current_dir_name() As Pointer In "libc:6"
Public Sub Main()
Dim pFunc As Pointer
Dim sWorkingDirectory As String
'get_current_dir_name
pFunc = Alloc(4097)
pFunc = get_current_dir_name()
sWorkingDirectory = Str@(pFunc)
Print sWorkingDirectory
Free(pFunc)
End
-----------------------
here i get correct results
but if i use Free(pFunc) i get signal #11
Example 3 ---------------------------
' Gambas module file
' Gambas module file
'char *getwd(char *buf);
Extern getwd(buf As Pointer) As Pointer In "libc:6"
Public Sub Main()
Dim pBuf As Pointer
Dim pFunc As Pointer
Dim sWorkingDirectory As String
'getwd
pBuf = Alloc(4097)
pFunc = Alloc(4097)
pFunc = getwd(pBuf)
sWorkingDirectory = Str@(pFunc)
' Free(pBuf)
Print sWorkingDirectory
Free(pFunc)
Print sWorkingDirectory
End
--------------------------
here the same with example1.
Free(pBuf)
Free(pFunc)
raises both signal #6
and
*** glibc detected *** Pointers5: double free or corruption (!prev):
0x09b0e4a8 ***
also
Print sWorkingDirectory
Free(pFunc)
Print sWorkingDirectory
first Print is correct
second after Free is not correct
/home/user/Documents/Development/Gambas3/Pointers5
�s&^@^@^@^@^@^@^@^@^@ocuments/Development/Gambas3/Pointers5
On Mon, 2011-01-10 at 01:16 +0200, Demosthenes Koptsis wrote:
> 1) Ok, i found why this function is dangerous.
> https://buildsecurityin.us-cert.gov/bsi/articles/knowledge/coding/760-BSI.html
> "Since the user cannot specify the length of the buffer passed to
> getwd(), use of this function is discouraged. The length of a pathname
> described in {PATH_MAX} is file system-dependent and may vary from one
> mount point to another, or might even be unlimited. It is possible to
> overflow this buffer in such a way as to cause applications to fail or
> possible system security violations."
>
> But here i make a test no real life project.
>
> 2) So, i found in /usr/include/linux/limits.h
> Line Number: 12
> #define PATH_MAX 4096 /* # chars in a path name including nul */
>
> is this the value?
>
> i tried this value in my example with no success, i make somewhere else
> the mistake.
>
> ---------------
> ' Gambas module file
> ' Gambas module file
> 'char *getwd(char *buf);
> Extern getwd(buf As Pointer) As Pointer In "libc:6"
>
> Public Sub Main()
>
> Dim pBuf As Pointer
> Dim pFunc As Pointer
> Dim sWorkingDirectory As String
>
> 'getwd
> pBuf = Alloc(4096)
> pFunc = Alloc(4096)
>
> pFunc = getwd(pBuf)
>
> sWorkingDirectory = Str@(pFunc)
> Free(pFunc)
> Free(pBuf)
> Print sWorkingDirectory
>
> End
> ---------------
>
> signal #6
>
> ----------------
> *** glibc detected *** Pointers5: double free or corruption (!prev):
> 0x0893f498 ***
> ======= Backtrace: =========
> /lib/tls/i686/cmov/libc.so.6(+0x6b591)[0x17b591]
> /lib/tls/i686/cmov/libc.so.6(+0x6cde8)[0x17cde8]
> /lib/tls/i686/cmov/libc.so.6(cfree+0x6d)[0x17fecd]
> Pointers5[0x805ed2e]
> Pointers5[0x8076a6a]
> Pointers5[0x8050a99]
> Pointers5[0x80513d8]
> Pointers5[0x80689cd]
> /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0x126bd6]
> Pointers5[0x804b151]
> ======= Memory map: ========
> 00110000-00263000 r-xp 00000000 08:04
> 2019753 /lib/tls/i686/cmov/libc-2.11.1.so
> 00263000-00264000 ---p 00153000 08:04
> 2019753 /lib/tls/i686/cmov/libc-2.11.1.so
> 00264000-00266000 r--p 00153000 08:04
> 2019753 /lib/tls/i686/cmov/libc-2.11.1.so
> 00266000-00267000 rw-p 00155000 08:04
> 2019753 /lib/tls/i686/cmov/libc-2.11.1.so
> 00267000-0026a000 rw-p 00000000 00:00 0
> 00428000-00438000 r-xp 00000000 08:04
> 1906544 /usr/local/lib/gambas3/gb.eval.so.0.0.0
> 00438000-00439000 r--p 0000f000 08:04
> 1906544 /usr/local/lib/gambas3/gb.eval.so.0.0.0
> 00439000-0043b000 rw-p 00010000 08:04
> 1906544 /usr/local/lib/gambas3/gb.eval.so.0.0.0
> 0043b000-0043c000 rw-p 00000000 00:00 0
> 0046e000-0046f000 r-xp 00000000 00:00 0 [vdso]
> 006c9000-006ce000 r-xp 00000000 08:04
> 688690 /usr/lib/libffi.so.5.0.10
> 006ce000-006cf000 ---p 00005000 08:04
> 688690 /usr/lib/libffi.so.5.0.10
> 006cf000-006d0000 r--p 00005000 08:04
> 688690 /usr/lib/libffi.so.5.0.10
> 006d0000-006d1000 rw-p 00006000 08:04
> 688690 /usr/lib/libffi.so.5.0.10
> 00739000-0075d000 r-xp 00000000 08:04
> 2019761 /lib/tls/i686/cmov/libm-2.11.1.so
> 0075d000-0075e000 r--p 00023000 08:04
> 2019761 /lib/tls/i686/cmov/libm-2.11.1.so
> 0075e000-0075f000 rw-p 00024000 08:04
> 2019761 /lib/tls/i686/cmov/libm-2.11.1.so
> 00aa5000-00ac2000 r-xp 00000000 08:04 1995027 /lib/libgcc_s.so.1
> 00ac2000-00ac3000 r--p 0001c000 08:04 1995027 /lib/libgcc_s.so.1
> 00ac3000-00ac4000 rw-p 0001d000 08:04 1995027 /lib/libgcc_s.so.1
> 00bca000-00bcc000 r-xp 00000000 08:04
> 2019759 /lib/tls/i686/cmov/libdl-2.11.1.so
> 00bcc000-00bcd000 r--p 00001000 08:04
> 2019759 /lib/tls/i686/cmov/libdl-2.11.1.so
> 00bcd000-00bce000 rw-p 00002000 08:04
> 2019759 /lib/tls/i686/cmov/libdl-2.11.1.so
> 00c25000-00c3a000 r-xp 00000000 08:04
> 2019772 /lib/tls/i686/cmov/libpthread-2.11.1.so
> 00c3a000-00c3b000 r--p 00014000 08:04
> 2019772 /lib/tls/i686/cmov/libpthread-2.11.1.so
> 00c3b000-00c3c000 rw-p 00015000 08:04
> 2019772 /lib/tls/i686/cmov/libpthread-2.11.1.so
> 00c3c000-00c3e000 rw-p 00000000 00:00 0
> 00dc3000-00dc7000 r-xp 00000000 08:04
> 1906676 /usr/local/lib/gambas3/gb.debug.so.0.0.0
> 00dc7000-00dc8000 r--p 00003000 08:04
> 1906676 /usr/local/lib/gambas3/gb.debug.so.0.0.0
> 00dc8000-00dc9000 rw-p 00004000 08:04
> 1906676 /usr/local/lib/gambas3/gb.debug.so.0.0.0
> 00eeb000-00f06000 r-xp 00000000 08:04 1996732 /lib/ld-2.11.1.so
> 00f06000-00f07000 r--p 0001a000 08:04 1996732 /lib/ld-2.11.1.so
> 00f07000-00f08000 rw-p 0001b000 08:04 1996732 /lib/ld-2.11.1.so
> 08048000-0807e000 r-xp 00000000 08:04 1785292 /usr/local/bin/gbx3
> 0807e000-0807f000 r--p 00035000 08:04 1785292 /usr/local/bin/gbx3
> 0807f000-08084000 rw-p 00036000 08:04 1785292 /usr/local/bin/gbx3
> 08084000-08086000 rw-p 00000000 00:00 0
> 08932000-08953000 rw-p 00000000 00:00 0 [heap]
> b7500000-b7521000 rw-p 00000000 00:00 0
> b7521000-b7600000 ---p 00000000 00:00 0
> b768a000-b76c9000 r--p 00000000 08:04
> 737484 /usr/lib/locale/en_US.utf8/LC_CTYPE
> b76c9000-b77e7000 r--p 00000000 08:04
> 737483 /usr/lib/locale/en_US.utf8/LC_COLLATE
> b77e7000-b77e9000 rw-p 00000000 00:00 0
> b77f1000-b77f2000 r--p 00000000 08:04
> 737500 /usr/lib/locale/en_US.utf8/LC_NUMERIC
> b77f2000-b77f3000 r--p 00000000 08:04
> 735917 /usr/lib/locale/en_US.utf8/LC_TIME
> b77f3000-b77f4000 r--p 00000000 08:04
> 735918 /usr/lib/locale/en_US.utf8/LC_MONETARY
> b77f4000-b77f5000 r--p 00000000 08:04
> 752258 /usr/lib/locale/en_US.utf8/LC_MESSAGES/SYS_LC_MESSAGES
> b77f5000-b77f6000 r--p 00000000 08:04
> 737523 /usr/lib/locale/en_US.utf8/LC_PAPER
> b77f6000-b77f7000 r--p 00000000 08:04
> 737455 /usr/lib/locale/en_US.utf8/LC_NAME
> b77f7000-b77f8000 r--p 00000000 08:04
> 735919 /usr/lib/locale/en_US.utf8/LC_ADDRESS
> b77f8000-b77f9000 r--p 00000000 08:04
> 735920 /usr/lib/locale/en_US.utf8/LC_TELEPHONE
> b77f9000-b77fa000 r--p 00000000 08:04
> 737398 /usr/lib/locale/en_US.utf8/LC_MEASUREMENT
> b77fa000-b7801000 r--s 00000000 08:04
> 1619066 /usr/lib/gconv/gconv-modules.cache
> b7801000-b7802000 r--p 00000000 08:04
> 735921 /usr/lib/locale/en_US.utf8/LC_IDENTIFICATION
> b7802000-b7806000 rw-p 00000000 00:00 0
> bfa53000-bfa68000 rw-p 00000000 00:00 0 [stack]
>
> ----------------
>
> i saw that the first line says
> *** glibc detected *** Pointers5: double free or corruption (!prev)
>
> and i rem the one Free().
>
> now i get
> --------------
> gbx3: warning: 1 allocation(s) non freed.
>
> ��m^@^@^@^@^@^@^@^@^@ocuments/Development/Gambas3/Pointers5
> --------------
>
> but no crash.
>
> i played with the values of Alloc and i found that in my case a 29 bytes
> are ok but with one Free() or else i get signal #11.
>
> is there a bug with
> Free(pBuf)
> Free(pFunc)
> ?
>
> 3) On the other hand, i have a general question.
> How to deal with char *pointers when their length is not known?
>
> 4) For example i try to use
> 'char *get_current_dir_name(void);
> Extern get_current_dir_name() As Pointer In "libc:6"
>
> again with no success.
>
> in manuall says:
> get_current_dir_name() will malloc(3) an array big enough to hold the
> absolute pathname of the current working directory.
>
> in my example:
>
> ---------------------
> ' Gambas module file
> 'char *get_current_dir_name(void);
> Extern get_current_dir_name() As Pointer In "libc:6"
>
> Public Sub Main()
>
> Dim pFunc As Pointer
> Dim sWorkingDirectory As String
>
> 'get_current_dir_name
> pFunc = Alloc(4096)
>
> pFunc = get_current_dir_name()
>
> sWorkingDirectory = Str@(pFunc)
> ' Free(pFunc)
> Print sWorkingDirectory
>
>
> End
> ---------------------
>
> if i use
> Free(pFunc)
>
> i get signal #11
>
> gambas3-svn3418, ubuntu 10.04
>
>
> On Sun, 2011-01-09 at 21:37 +0100, Benoît Minisini wrote:
> > > so what can i do for
> > >
> > > char *getwd(char *buf);
> > >
> > > ?
> > >
> >
> > You must know C programming first, and understand how getwd works. By typing
> > 'man getwd', you will learn that you must not use that function, it is
> > dangerous and so deprecated.
> >
> > Then you will read that buf must be a pointer to a memory allocation of
> > PATH_MAX bytes. You must find the value of this constant inside the standard C
> > header files.
> >
> > SizeOf(gb.String) is 4 (on 32 bits system). It is the number of bytes used by
> > Gambas to store a pointer to a Gambas string. This is the reason why you get a
> > crash.
> >
> > Regards,
> >
>
--
Regards,
Demosthenes
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