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| <title>Debugging with GDB: i386</title> |
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| <a name="x86-Architecture_002dspecific-Issues"></a> |
| <h4 class="subsection">21.4.2 x86 Architecture-specific Issues</h4> |
| |
| <dl compact="compact"> |
| <dt><code>set struct-convention <var>mode</var></code></dt> |
| <dd><a name="index-set-struct_002dconvention"></a> |
| <a name="index-struct-return-convention"></a> |
| <a name="index-struct_002funion-returned-in-registers"></a> |
| <p>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. |
| </p> |
| </dd> |
| <dt><code>show struct-convention</code></dt> |
| <dd><a name="index-show-struct_002dconvention"></a> |
| <p>Show the current setting of the convention to return <code>struct</code>s |
| from functions. |
| </p></dd> |
| </dl> |
| |
| |
| <a name="Intel_0028R_0029-Memory-Protection-Extensions-_0028MPX_0029_002e"></a> |
| <h4 class="subsubsection">21.4.2.1 Intel(R) <em>Memory Protection Extensions</em> (MPX).</h4> |
| <a name="index-Intel_0028R_0029-Memory-Protection-Extensions-_0028MPX_0029_002e"></a> |
| |
| <p>Memory Protection Extension (MPX) adds the bound registers ‘<samp>BND0</samp>’ |
| <a name="DOCF16" href="#FOOT16"><sup>16</sup></a> through ‘<samp>BND3</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> |
| <p>‘<samp>BND0</samp>’ through ‘<samp>BND3</samp>’ are represented in <small>GDB</small> as ‘<samp>bnd0raw</samp>’ |
| through ‘<samp>bnd3raw</samp>’. Pseudo registers ‘<samp>bnd0</samp>’ through ‘<samp>bnd3</samp>’ |
| display the upper bound performing the complement of one operation on the |
| upper bound value, i.e. when upper bound in ‘<samp>bnd0raw</samp>’ is 0 in the |
| <small>GDB</small> ‘<samp>bnd0</samp>’ it will be <code>0xfff…</code>. In this sense it |
| can also be noted that the upper bounds are inclusive. |
| </p> |
| <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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> bnd0raw = {0x32, 0xffffffff8e} |
| bnd0 = {lbound = 0x32, ubound = 0x71} : size 64 |
| </pre></div> |
| |
| <p>This way the raw value can be accessed via bnd0raw…bnd3raw. Any |
| change on bnd0…bnd3 or bnd0raw…bnd3raw is reflect on its |
| counterpart. When the bnd0…bnd3 registers are displayed via |
| Python, the display includes the memory size, in bits, accessible to |
| the pointer. |
| </p> |
| <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. <small>GDB</small> provides commands for this purpose: |
| </p> |
| <dl compact="compact"> |
| <dt><code>show mpx bound <var>pointer</var></code></dt> |
| <dd><a name="index-show-mpx-bound"></a> |
| <p>Display bounds of the given <var>pointer</var>. |
| </p> |
| </dd> |
| <dt><code>set mpx bound <var>pointer</var>, <var>lbound</var>, <var>ubound</var></code></dt> |
| <dd><a name="index-set-mpx-bound"></a> |
| <p>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. |
| </p></dd> |
| </dl> |
| |
| <div class="footnote"> |
| <hr> |
| <h4 class="footnotes-heading">Footnotes</h4> |
| |
| <h3><a name="FOOT16" href="#DOCF16">(16)</a></h3> |
| <p>The register named with capital letters represent the architecture |
| registers.</p> |
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