On 19 January 2017 at 12:02, Ard Biesheuvel [off-list ref] wrote:

quoted

The modversion symbol CRCs are emitted as ELF symbols, which allows us to
easily populate the kcrctab sections by relying on the linker to associate
each kcrctab slot with the correct value.

This has a couple of downsides:
- On architectures that support runtime relocation, a R_<arch>_RELATIVE
  relocation entry is emitted for each CRC value, which identifies it as
  a quantity that requires fixing up based on the actual runtime load
  offset of the kernel. This results in corrupted CRCs unless we
  explicitly undo the fixup (and this is currently being handled in the
  core module code),
- On 64 bit architectures, such runtime relocation entries take up 24
  bytes of __init space each, resulting in a x8 overhead in [uncompressed]
  kernel size for CRCs,
- The use of ELF symbols to represent things other than virtual addresses
  is poorly supported (and mostly untested) in GNU binutils.

So avoid these issues by redefining the kcrctab entries as signed relative
offsets pointing to the actual CRC value stored elsewhere in the kernel
image (or in the module). This removes all the ELF hackery involving
symbols and relocations, at the expense of 4 additional bytes of space per
CRC on 32-bit architectures. (On 64-bit architectures, each 8 byte CRC
symbol reference is replaced by a 4 byte relative offset and the 4 byte
CRC value elsewhere in the image)

Note that this mostly reverts commit d4703aefdbc8 ("module: handle ppc64
relocating kcrctabs when CONFIG_RELOCATABLE=y")

Signed-off-by: Ard Biesheuvel <redacted>
---
v2: update modpost as well, so that genksyms no longer has to emit symbols
    for both the actual CRC value and the reference to where it is stored
    in the image

Annoyingly, the following is required on top of this patch to ensure
that we only subtract the section VMA from the symbol value in modpost
if we are dealing with a fully linked file. This is necessary since,
as it turns out, partially linked ELF objects may have non-zero
section VMAs (for whatever reason).

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index d69f3ceae19e..75b0dac1cc11 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c

@@ -624,7 +624,8 @@
                if (sym->st_shndx != SHN_UNDEF)
                        crcp = (void *)info->hdr + sym->st_value +
                               info->sechdrs[sym->st_shndx].sh_offset -
-                              info->sechdrs[sym->st_shndx].sh_addr;
+                              (hdr->e_type != ET_REL ?