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(stabs.info)ELF Linker Relocation

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Having the Linker Relocate Stabs in ELF
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   This section describes some Sun hacks for Stabs in ELF; it does not
apply to COFF or SOM.

   To keep linking fast, you don't want the linker to have to relocate
very many stabs.  Making sure this is done for `N_SLINE', `N_RBRAC',
and `N_LBRAC' 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 `n_value' field (functions
and static variables) relative to the source file.  For the `N_SO'
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
`Bbss.bss' for the bss section, `Ddata.data' for the data section, and
`Drodata.rodata' 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, Note: Stab Section Transformations.  GCC does
not provide these symbols; it instead relies on the stabs getting
relocated.  Thus addresses which would normally be relative to
`Bbss.bss', 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 `S' symbol descriptor stabs (Note: Statics) and functions
(Note: Procedures).  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 `Ttext.text' symbol, analogous
to the `Bbss.bss', etc., symbols.  I recommend this rather than using a
zero value and getting the address from the ELF symbols.

   Finding the correct `Bbss.bss', etc., symbol is difficult, because
the linker simply concatenates the `.stab' sections from each `.o' file
without including any information about which part of a `.stab' section
comes from which `.o' file.  The way GDB does this is to look for an
ELF `STT_FILE' symbol which has the same name as the last component of
the file name from the `N_SO' symbol in the stabs (for example, if the
file name is `../../gdb/main.c', it looks for an ELF `STT_FILE' symbol
named `main.c').  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 `Bbss.bss' 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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