101 lines
5.8 KiB
HTML
101 lines
5.8 KiB
HTML
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<html lang="en">
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<head>
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<title>ELF Linker Relocation - STABS</title>
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<meta name="description" content="STABS">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Stab-Sections.html#Stab-Sections" title="Stab Sections">
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<link rel="prev" href="Stab-Section-Basics.html#Stab-Section-Basics" title="Stab Section Basics">
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<!--
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Copyright (C) 1992-2019 Free Software Foundation, Inc.
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Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon,
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and David MacKenzie.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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<body>
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<div class="node">
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<a name="ELF-Linker-Relocation"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Stab-Section-Basics.html#Stab-Section-Basics">Stab Section Basics</a>,
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Up: <a rel="up" accesskey="u" href="Stab-Sections.html#Stab-Sections">Stab Sections</a>
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<hr>
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</div>
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<h3 class="appendixsec">F.2 Having the Linker Relocate Stabs in ELF</h3>
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<p>This section describes some Sun hacks for Stabs in ELF; it does not
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apply to COFF or SOM. While <span class="sc">gdb</span> no longer supports this hack
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for Sun Stabs in ELF, this section is kept to document the issue.
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<p>To keep linking fast, you don't want the linker to have to relocate very
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many stabs. Making sure this is done for <code>N_SLINE</code>,
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<code>N_RBRAC</code>, and <code>N_LBRAC</code> stabs is the most important thing
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(see the descriptions of those stabs for more information). But Sun's
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stabs in ELF has taken this further, to make all addresses in the
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<code>n_value</code> field (functions and static variables) relative to the
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source file. For the <code>N_SO</code> symbol itself, Sun simply omits the
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address. To find the address of each section corresponding to a given
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source file, the compiler puts out symbols giving the address of each
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section for a given source file. Since these are ELF (not stab)
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symbols, the linker relocates them correctly without having to touch the
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stabs section. They are named <code>Bbss.bss</code> for the bss section,
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<code>Ddata.data</code> for the data section, and <code>Drodata.rodata</code> for
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the rodata section. For the text section, there is no such symbol (but
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there should be, see below). For an example of how these symbols work,
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See <a href="Stab-Section-Transformations.html#Stab-Section-Transformations">Stab Section Transformations</a>. GCC does not provide these symbols;
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it instead relies on the stabs getting relocated. Thus addresses which
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would normally be relative to <code>Bbss.bss</code>, etc., are already
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relocated. The Sun linker provided with Solaris 2.2 and earlier
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relocates stabs using normal ELF relocation information, as it would do
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for any section. Sun has been threatening to kludge their linker to not
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do this (to speed up linking), even though the correct way to avoid
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having the linker do these relocations is to have the compiler no longer
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output relocatable values. Last I heard they had been talked out of the
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linker kludge. See Sun point patch 101052-01 and Sun bug 1142109. With
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the Sun compiler this affects ‘<samp><span class="samp">S</span></samp>’ symbol descriptor stabs
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(see <a href="Statics.html#Statics">Statics</a>) and functions (see <a href="Procedures.html#Procedures">Procedures</a>). In the latter
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case, to adopt the clean solution (making the value of the stab relative
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to the start of the compilation unit), it would be necessary to invent a
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<code>Ttext.text</code> symbol, analogous to the <code>Bbss.bss</code>, etc.,
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symbols. I recommend this rather than using a zero value and getting
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the address from the ELF symbols.
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<p>Finding the correct <code>Bbss.bss</code>, etc., symbol is difficult, because
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the linker simply concatenates the <code>.stab</code> sections from each
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<samp><span class="file">.o</span></samp> file without including any information about which part of a
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<code>.stab</code> section comes from which <samp><span class="file">.o</span></samp> file. The way GDB use to
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do this is to look for an ELF <code>STT_FILE</code> symbol which has the same
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name as the last component of the file name from the <code>N_SO</code> symbol
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in the stabs (for example, if the file name is <samp><span class="file">../../gdb/main.c</span></samp>,
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it looks for an ELF <code>STT_FILE</code> symbol named <code>main.c</code>). This
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loses if different files have the same name (they could be in different
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directories, a library could have been copied from one system to
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another, etc.). It would be much cleaner to have the <code>Bbss.bss</code>
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symbols in the stabs themselves. Having the linker relocate them there
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is no more work than having the linker relocate ELF symbols, and it
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solves the problem of having to associate the ELF and stab symbols.
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However, no one has yet designed or implemented such a scheme.
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</body></html>
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