[Forwarding this to linaro-dev too ... should have sent it there in the first place! ---Dave]

---------- Forwarded message ---------- From: Dave Martin dave.martin@linaro.org Date: Fri, Jan 28, 2011 at 11:52 AM Subject: Generating ancilliary sections with gas To: linaro-toolchain linaro-toolchain@lists.linaro.org

Hi all,

With gas, does anyone know of a way to create a section whose name is based on that of the current section?

The specific requirement is to be able to define a generic macro like the example "fixup" below, whose purpose is to record ancilliary data related to some other section.  To illustrate:

.macro fixup 100@ :    .pushsection fixup<current section name>, "a"        .long 100@b    .popsection .endm

.text ...    fixup    .long sym1 ...

.section .other, "ax" ...    fixup    .long sym2

The linux kernel uses a technique just like this for patching SMP kernels at bootup to work on uniprocessor platforms (when CONFIG_SMP_ON_UP is enabled), resulting in code looking something like this:

void exit __attribute__ (( __section__ (".text.exit") )) {    ...    asm(        ...        FIXUP("something")        ...    ); }

Note that the inline asm may actually come out of a generic header file rather than being explitly written for this invocation.  So it may have to be truly generic.

Is far as I have been able to determine, it's not possible to generate sections named based on the current section.  In practice, the kernel puts all the fixups into a single section.

The downside of this is that when sections are selectively discarded at link time (which in general may happen -- for example, Linux discards the "module exit" code for drivers which are built into the kernel and therefore never exit) there is no way to selectively discard the related fixup entries.  Currently the only solution is to include all the module exit code in the image and discard it at run-time when the kernel boots.  This is obviously wasteful. Attempting to discard that code at like time results in a link error, since fixups refer to the removed sections.

Of course, the "fixup" macro could be given an extra parameter to name the containing section, but the macro can then no longer be called in a generic way: all the calls to that macro must be manually (and buggily) maintained to ensure that the referenced section name is correct, some object post-processing must be done before linking, and/or a tool must be created to implement the missing assembler functionality.  Unfortunately, such solutions are likely to be too fragile or complex to make it upstream.

It's interesting to note that the same problem will apply for any section containing ancilliary data for another section.  In particular, it looks like either the ABI or the assembler has had to grow a special-case workaround for this in order to support exception unwind information sections generated by .fnstart ... .fnend in a sane way: the unwind information sections get called .ARM.ex{idx,tab} for .text, and .ARM.ex{idx,tab}<section> for any other section.  As a consequence, link-time discarding can handle this information properly, but IMHO this is a bit of a cheat and admits the general need to create sections with names based transparently on those of other sections, without satisfying that need.  .popsection is also an example of such a cheat: most other aspects of assmbler state still cannot be saved and restored.

In general, it would be useful if gas supported some general reflective abilities: i.e., the ability to query the current assembler state (section, subsection, active instruction set, active macro mode, etc.) and/or the ability to wrap or hook existing pseudo-ops.  For example, the above problem would almost certainly solvable using assembler macros (albeit painfully) if wrapper macros could be defined for the section manipulation directives (section, .text, .data, .bss, .pushsection, .popsection, .previous).  However, supporting some magic macro parameters reflecting the assembler state would be a lot simpler.

As an example of the kind of behaviour I think would be useful, the macro argument qualifier could be extended to allow macros to query the assembler state in a backwards-compatible way; something like:

.macro fixup base_section:gas_current_section_name, old_altmacro:gas_macro_mode    .altmacro    LOCAL fixup_location

fixup_location:    .pushsection \base_section().fixup        .long 100@b    .popsection

\old_altmacro .endm

Existing assembler code will continue to work just fine with this approach.

Note how this also enables a local label to be generated hygenically, by making it possible to save and restore the macro mode.  Otherwise, .altmacro (and hence LOCAL) is hard to use safely, since the initial macro mode is unknown and can't be restored.

Any thoughts / comments?

Cheers. ---Dave