I think I have a testcase where ld --gc-sections erroneously
discards a section. I'd appreciate if someone will take
a look at it.

You can download it from
http://busybox.net/~vda/ksymtab_bug.tar.bz2

This testcase is adapted from Linux kernel build.

Kernel image is linked using custom linker script. It must contain some
special sections needed for dynamic loading of external modules at runtime.
One such section is __ksymtab.

It seems that in this testcase ld does not follow instructions
given to it in custom linker script (arch_x86_64_kernel_vmlinux.lds).

For some reason __ksymtab___const_udelay and __ksymtab_synchronize_irq
symbols are not included into zz_vmlinux_bad, while other __ksymtab_*
symbols are included, as intended (for example, __ksymtab___udelay).

--print-gc-sections does not report them as discarded,
and they shouldn't be, because they are coming from __ksymtab input
sections, and arch_x86_64_kernel_vmlinux.lds has KEEP directive for those:
__ksymtab : {  KEEP(*(__ksymtab)) }

[in fact, zz_discarded.lst does not indicate any of __ksymtab sections as discarded]

__ksymtab___const_udelay is in arch_x86_64_lib_lib.a,
__ksymtab_synchronize_irq is in kernel_built-in.o,
so it doesn't seem to be caused by .a versus .o difference.

__ksymtab___const_udelay and __ksymtab_synchronize_irq
are specific to this particular set of .o/.a files.
With slightly different kernel .config, other __ksymtab_XXX sections
will be erroneously discarded. There is no apparent pattern
in which ones will be affected.

# x86_64-pc-linux-gnu-ld -v
GNU ld (GNU Binutils) 2.18

# Bad link:
x86_64-pc-linux-gnu-ld -m elf_x86_64 --gc-sections --print-gc-sections \
--build-id \
-o zz_vmlinux_bad \
-Map zz_link_bad.map -T arch_x86_64_kernel_vmlinux.lds \
\
arch_x86_64_kernel_head.o arch_x86_64_kernel_head64.o \
arch_x86_64_kernel_init_task.o init_built-in.o \
--start-group \
usr_built-in.o arch_x86_64_kernel_built-in.o arch_x86_64_mm_built-in.o \
arch_x86_64_crypto_built-in.o arch_x86_64_vdso_built-in.o \
arch_x86_64_ia32_built-in.o kernel_built-in.o mm_built-in.o \
fs_built-in.o ipc_built-in.o security_built-in.o crypto_built-in.o \
block_built-in.o lib_lib.a arch_x86_64_lib_lib.a lib_built-in.o \
arch_x86_64_lib_built-in.o drivers_built-in.o sound_built-in.o \
arch_x86_64_pci_built-in.o net_built-in.o \
--end-group \
.tmp_kallsyms3.o \
  2>zz_discarded.lst

# Here you will see __ksymtab___udelay but no __ksymtab___const_udelay
objdump -x zz_vmlinux_bad | grep 'ksymtab[a-z_]*udelay'
echo
objdump -x zz_vmlinux_bad | grep '_synchronize_irq'
echo

# If one removes --gc-sections, link works:
x86_64-pc-linux-gnu-ld -m elf_x86_64 \
--build-id \
-o zz_vmlinux_good \
-Map zz_link_good.map -T arch_x86_64_kernel_vmlinux.lds \
\
arch_x86_64_kernel_head.o arch_x86_64_kernel_head64.o \
arch_x86_64_kernel_init_task.o init_built-in.o \
--start-group \
usr_built-in.o arch_x86_64_kernel_built-in.o arch_x86_64_mm_built-in.o \
arch_x86_64_crypto_built-in.o arch_x86_64_vdso_built-in.o \
arch_x86_64_ia32_built-in.o kernel_built-in.o mm_built-in.o \
fs_built-in.o ipc_built-in.o security_built-in.o crypto_built-in.o \
block_built-in.o lib_lib.a arch_x86_64_lib_lib.a lib_built-in.o \
arch_x86_64_lib_built-in.o drivers_built-in.o sound_built-in.o \
arch_x86_64_pci_built-in.o net_built-in.o \
--end-group \
.tmp_kallsyms3.o

# Here you will see both __ksymtab___udelay and __ksymtab___const_udelay
objdump -x zz_vmlinux_good | grep 'ksymtab[a-z_]*udelay'
echo
objdump -x zz_vmlinux_good | grep '_synchronize_irq'
echo


Sample output:

ffffffff806e0f00 l     O __ksymtab      0000000000000010 __ksymtab___udelay
(__ksymtab___const_udelay should be listed too, but it isnt)

ffffffff806eb590 l     O __kcrctab      0000000000000008 __kcrctab_synchronize_irq
ffffffff806f4b20 l     O __ksymtab_strings      0000000000000010
__kstrtab_synchronize_irq
00000000e523ad75 g       *ABS*  0000000000000000 __crc_synchronize_irq

ffffffff806e7050 l     O __ksymtab      0000000000000010 __ksymtab___const_udelay
ffffffff806e7060 l     O __ksymtab      0000000000000010 __ksymtab___udelay

ffffffff806f16f0 l     O __kcrctab      0000000000000008 __kcrctab_synchronize_irq
ffffffff806e3b90 l     O __ksymtab      0000000000000010 __ksymtab_synchronize_irq
ffffffff806fac80 l     O __ksymtab_strings      0000000000000010
__kstrtab_synchronize_irq
00000000e523ad75 g       *ABS*  0000000000000000 __crc_synchronize_irq