• Russell King's avatar
    ARM: fix vmlinux.lds.S discarding sections · 6760b109
    Russell King authored
    We are seeing linker errors caused by sections being discarded, despite
    the linker script trying to keep them.  The result is (eg):
    
    `.exit.text' referenced in section `.alt.smp.init' of drivers/built-in.o: defined in discarded section `.exit.text' of drivers/built-in.o
    `.exit.text' referenced in section `.alt.smp.init' of net/built-in.o: defined in discarded section `.exit.text' of net/built-in.o
    
    This is the relevent part of the linker script (reformatted to make it
    clearer):
    | SECTIONS
    | {
    | /*
    | * unwind exit sections must be discarded before the rest of the
    | * unwind sections get included.
    | */
    | /DISCARD/ : {
    | *(.ARM.exidx.exit.text)
    | *(.ARM.extab.exit.text)
    | }
    | ...
    | .exit.text : {
    | *(.exit.text)
    | *(.memexit.text)
    | }
    | ...
    | /DISCARD/ : {
    | *(.exit.text)
    | *(.memexit.text)
    | *(.exit.data)
    | *(.memexit.data)
    | *(.memexit.rodata)
    | *(.exitcall.exit)
    | *(.discard)
    | *(.discard.*)
    | }
    | }
    
    Now, this is what the linker manual says about discarded output sections:
    
    |    The special output section name `/DISCARD/' may be used to discard
    | input sections.  Any input sections which are assigned to an output
    | section named `/DISCARD/' are not included in the output file.
    
    No questions, no exceptions. It doesn't say "unless they are listed
    before the /DISCARD/ section." Now, this is what asn-generic/vmlinux.lds.S
    says:
    | /*
    |  * Default discarded sections.
    |  *
    |  * Some archs want to discard exit text/data at runtime rather than
    |  * link time due to cross-section references such as alt instructions,
    |  * bug table, eh_frame, etc. DISCARDS must be the last of output
    |  * section definitions so that such archs put those in earlier section
    |  * definitions.
    |  */
    
    And guess what - the list _always_ includes .exit.text etc.
    
    Now, what's actually happening is that the linker is reading the script,
    and it finds the first /DISCARD/ output section at the beginning of the
    script. It continues reading the script, and finds the 'DISCARD' macro
    at the end, which having been postprocessed results in another
    /DISCARD/ output section. As the linker already contains the earlier
    /DISCARD/ output section, it adds it to that existing section, so it
    effectively is placed at the start. This can be seen by using the -M
    option to ld:
    
    | Linker script and memory map
    |
    |                 0xc037c080                jiffies = jiffies_64
    |
    | /DISCARD/
    |  *(.ARM.exidx.exit.text)
    |  *(.ARM.extab.exit.text)
    |  *(.exit.text)
    |  *(.memexit.text)
    |  *(.exit.data)
    |  *(.memexit.data)
    |  *(.memexit.rodata)
    |  *(.exitcall.exit)
    |  *(.discard)
    |  *(.discard.*)
    |
    |                 0xc0008000                . = 0xc0008000
    |
    | .head.text      0xc0008000      0x1d0
    |                 0xc0008000                _text = .
    |  *(.head.text)
    |  .head.text     0xc0008000      0x1d0 arch/arm/kernel/head.o
    |                 0xc0008000                stext
    |
    | .text           0xc0008200   0x2d78d0
    |                 0xc0008200                _stext = .
    |                 0xc0008200                __exception_text_start = .
    |  *(.exception.text)
    |  .exception.text
    | ...
    
    As you can see, all the discarded sections are grouped together - and
    as a result of it being the first output section, they all appear before
    any other section.
    
    The result is that not only is the unwind information discarded (as
    intended), but also the .exit.text, despite us wanting to have the
    .exit.text preserved.
    
    We can't move the unwind information elsewhere, because it'll then be
    included even when we do actually discard the .exit.text (and similar)
    sections.
    
    So, work around this by avoiding the generic DISCARDS macro, and instead
    conditionalize the sections to be discarded ourselves.  This avoids the
    ambiguity in how the linker assigns input sections to output sections,
    making our script less dependent on undocumented linker behaviour.
    Reported-by: default avatarRob Herring <robherring2@gmail.com>
    Tested-by: default avatarMark Brown <broonie@opensource.wolfsonmicro.com>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
    6760b109
vmlinux.lds.S 5.83 KB