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| <h4 class="subsection">21.4.2 x86 Architecture-specific Issues</h4> |
| |
| <dl> |
| <dt><code>set struct-convention </code><var>mode</var><dd><a name="index-set-struct_002dconvention-1606"></a><a name="index-struct-return-convention-1607"></a><a name="index-struct_002funion-returned-in-registers-1608"></a>Set the convention used by the inferior to return <code>struct</code>s and |
| <code>union</code>s from functions to <var>mode</var>. Possible values of |
| <var>mode</var> are <code>"pcc"</code>, <code>"reg"</code>, and <code>"default"</code> (the |
| default). <code>"default"</code> or <code>"pcc"</code> means that <code>struct</code>s |
| are returned on the stack, while <code>"reg"</code> means that a |
| <code>struct</code> or a <code>union</code> whose size is 1, 2, 4, or 8 bytes will |
| be returned in a register. |
| |
| <br><dt><code>show struct-convention</code><dd><a name="index-show-struct_002dconvention-1609"></a>Show the current setting of the convention to return <code>struct</code>s |
| from functions. |
| </dl> |
| |
| <h5 class="subsubsection">21.4.2.1 Intel <dfn>Memory Protection Extensions</dfn> (MPX).</h5> |
| |
| <p><a name="index-Intel-Memory-Protection-Extensions-_0028MPX_0029_002e-1610"></a> |
| Memory Protection Extension (MPX) adds the bound registers ‘<samp><span class="samp">BND0</span></samp>’ |
| <a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> through ‘<samp><span class="samp">BND3</span></samp>’. Bound registers store a pair of 64-bit values |
| which are the lower bound and upper bound. Bounds are effective addresses or |
| memory locations. The upper bounds are architecturally represented in 1's |
| complement form. A bound having lower bound = 0, and upper bound = 0 |
| (1's complement of all bits set) will allow access to the entire address space. |
| |
| <p>‘<samp><span class="samp">BND0</span></samp>’ through ‘<samp><span class="samp">BND3</span></samp>’ are represented in <span class="sc">gdb</span> as ‘<samp><span class="samp">bnd0raw</span></samp>’ |
| through ‘<samp><span class="samp">bnd3raw</span></samp>’. Pseudo registers ‘<samp><span class="samp">bnd0</span></samp>’ through ‘<samp><span class="samp">bnd3</span></samp>’ |
| display the upper bound performing the complement of one operation on the |
| upper bound value, i.e. when upper bound in ‘<samp><span class="samp">bnd0raw</span></samp>’ is 0 in the |
| <span class="sc">gdb</span> ‘<samp><span class="samp">bnd0</span></samp>’ it will be <code>0xfff...</code>. In this sense it |
| can also be noted that the upper bounds are inclusive. |
| |
| <p>As an example, assume that the register BND0 holds bounds for a pointer having |
| access allowed for the range between 0x32 and 0x71. The values present on |
| bnd0raw and bnd registers are presented as follows: |
| |
| <pre class="smallexample"> bnd0raw = {0x32, 0xffffffff8e} |
| bnd0 = {lbound = 0x32, ubound = 0x71} : size 64 |
| </pre> |
| <p>This way the raw value can be accessed via bnd0raw<small class="dots">...</small>bnd3raw. Any |
| change on bnd0<small class="dots">...</small>bnd3 or bnd0raw<small class="dots">...</small>bnd3raw is reflect on its |
| counterpart. When the bnd0<small class="dots">...</small>bnd3 registers are displayed via |
| Python, the display includes the memory size, in bits, accessible to |
| the pointer. |
| |
| <p>Bounds can also be stored in bounds tables, which are stored in |
| application memory. These tables store bounds for pointers by specifying |
| the bounds pointer's value along with its bounds. Evaluating and changing |
| bounds located in bound tables is therefore interesting while investigating |
| bugs on MPX context. <span class="sc">gdb</span> provides commands for this purpose: |
| |
| <dl> |
| <dt><code>show mpx bound </code><var>pointer</var><dd><a name="index-show-mpx-bound-1611"></a>Display bounds of the given <var>pointer</var>. |
| |
| <br><dt><code>set mpx bound </code><var>pointer</var><code>, </code><var>lbound</var><code>, </code><var>ubound</var><dd><a name="index-set-mpx-bound-1612"></a>Set the bounds of a pointer in the bound table. |
| This command takes three parameters: <var>pointer</var> is the pointers |
| whose bounds are to be changed, <var>lbound</var> and <var>ubound</var> are new values |
| for lower and upper bounds respectively. |
| </dl> |
| |
| <p>When you call an inferior function on an Intel MPX enabled program, |
| GDB sets the inferior's bound registers to the init (disabled) state |
| before calling the function. As a consequence, bounds checks for the |
| pointer arguments passed to the function will always pass. |
| |
| <p>This is necessary because when you call an inferior function, the |
| program is usually in the middle of the execution of other function. |
| Since at that point bound registers are in an arbitrary state, not |
| clearing them would lead to random bound violations in the called |
| function. |
| |
| <p>You can still examine the influence of the bound registers on the |
| execution of the called function by stopping the execution of the |
| called function at its prologue, setting bound registers, and |
| continuing the execution. For example: |
| |
| <pre class="smallexample"> $ break *upper |
| Breakpoint 2 at 0x4009de: file i386-mpx-call.c, line 47. |
| $ print upper (a, b, c, d, 1) |
| Breakpoint 2, upper (a=0x0, b=0x6e0000005b, c=0x0, d=0x0, len=48).... |
| $ print $bnd0 |
| {lbound = 0x0, ubound = ffffffff} : size -1 |
| </pre> |
| <p>At this last step the value of bnd0 can be changed for investigation of bound |
| violations caused along the execution of the call. In order to know how to |
| set the bound registers or bound table for the call consult the ABI. |
| |
| <div class="footnote"> |
| <hr> |
| <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> The register named with capital letters represent the architecture |
| registers.</p> |
| |
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