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When building GDB using Ubuntu 20.04's system libgmp and compiler,
running the "maintenance selftest" command triggers the following error:
| Running selftest gdb_mpq_write_fixed_point.
| *** stack smashing detected ***: terminated
| [1] 1092790 abort (core dumped) ./gdb gdb
This happens while trying to construct an mpq_t object (a rational)
from two integers representing the numerator and denominator.
In our test, the numerator is -8, and the denominator is 1.
The problem was that the rational was constructed using the wrong
function. This is what we were doing prior to this patch:
mpq_set_ui (v.val, numerator, denominator);
The 'u' in "ui" stands for *unsigned*, which is wrong because
numerator and denominator's type is "int".
As a result of the above, instead of getting a rational value of -8,
we get a rational with a very large positive value (gmp_printf
says "18446744073709551608").
From there, the test performs an operation which is expected to
write this value into a buffer which was not dimensioned to fit
such a number, thus leading GMP into a buffer overflow.
This was verified by applying the formula that GMP's documentation
gives for the required memory buffer size needed during export:
| When an application is allocating space itself the required size can
| be determined with a calculation like the following. Since
| mpz_sizeinbase always returns at least 1, count here will be at
| least one, which avoids any portability problems with malloc(0),
| though if z is zero no space at all is actually needed (or written).
|
| numb = 8*size - nail;
| count = (mpz_sizeinbase (z, 2) + numb-1) / numb;
| p = malloc (count * size);
With the very large number, mpz_sizeinbase returns 66 and thus
the malloc size becomes 16 bytes instead of the 8 we allocated.
This patch fixes the issue by using the correct "set" function.
gdb/ChangeLog:
* unittests/gmp-utils-selftests.c (write_fp_test): Use mpq_set_si
instead of mpq_set_ui to initialize our GMP rational.
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.