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| <h3 class="appendixsec">F.2 Having the Linker Relocate Stabs in ELF</h3> |
| |
| <p>This section describes some Sun hacks for Stabs in ELF; it does not |
| apply to COFF or SOM. While <span class="sc">gdb</span> no longer supports this hack |
| for Sun Stabs in ELF, this section is kept to document the issue. |
| |
| <p>To keep linking fast, you don't want the linker to have to relocate very |
| many stabs. Making sure this is done for <code>N_SLINE</code>, |
| <code>N_RBRAC</code>, and <code>N_LBRAC</code> stabs is the most important thing |
| (see the descriptions of those stabs for more information). But Sun's |
| stabs in ELF has taken this further, to make all addresses in the |
| <code>n_value</code> field (functions and static variables) relative to the |
| source file. For the <code>N_SO</code> symbol itself, Sun simply omits the |
| address. To find the address of each section corresponding to a given |
| source file, the compiler puts out symbols giving the address of each |
| section for a given source file. Since these are ELF (not stab) |
| symbols, the linker relocates them correctly without having to touch the |
| stabs section. They are named <code>Bbss.bss</code> for the bss section, |
| <code>Ddata.data</code> for the data section, and <code>Drodata.rodata</code> for |
| the rodata section. For the text section, there is no such symbol (but |
| there should be, see below). For an example of how these symbols work, |
| See <a href="Stab-Section-Transformations.html#Stab-Section-Transformations">Stab Section Transformations</a>. GCC does not provide these symbols; |
| it instead relies on the stabs getting relocated. Thus addresses which |
| would normally be relative to <code>Bbss.bss</code>, etc., are already |
| relocated. The Sun linker provided with Solaris 2.2 and earlier |
| relocates stabs using normal ELF relocation information, as it would do |
| for any section. Sun has been threatening to kludge their linker to not |
| do this (to speed up linking), even though the correct way to avoid |
| having the linker do these relocations is to have the compiler no longer |
| output relocatable values. Last I heard they had been talked out of the |
| linker kludge. See Sun point patch 101052-01 and Sun bug 1142109. With |
| the Sun compiler this affects ‘<samp><span class="samp">S</span></samp>’ symbol descriptor stabs |
| (see <a href="Statics.html#Statics">Statics</a>) and functions (see <a href="Procedures.html#Procedures">Procedures</a>). In the latter |
| case, to adopt the clean solution (making the value of the stab relative |
| to the start of the compilation unit), it would be necessary to invent a |
| <code>Ttext.text</code> symbol, analogous to the <code>Bbss.bss</code>, etc., |
| symbols. I recommend this rather than using a zero value and getting |
| the address from the ELF symbols. |
| |
| <p>Finding the correct <code>Bbss.bss</code>, etc., symbol is difficult, because |
| the linker simply concatenates the <code>.stab</code> sections from each |
| <samp><span class="file">.o</span></samp> file without including any information about which part of a |
| <code>.stab</code> section comes from which <samp><span class="file">.o</span></samp> file. The way GDB use to |
| do this is to look for an ELF <code>STT_FILE</code> symbol which has the same |
| name as the last component of the file name from the <code>N_SO</code> symbol |
| in the stabs (for example, if the file name is <samp><span class="file">../../gdb/main.c</span></samp>, |
| it looks for an ELF <code>STT_FILE</code> symbol named <code>main.c</code>). This |
| loses if different files have the same name (they could be in different |
| directories, a library could have been copied from one system to |
| another, etc.). It would be much cleaner to have the <code>Bbss.bss</code> |
| symbols in the stabs themselves. Having the linker relocate them there |
| is no more work than having the linker relocate ELF symbols, and it |
| solves the problem of having to associate the ELF and stab symbols. |
| However, no one has yet designed or implemented such a scheme. |
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