ELF section not being linked

Question

I'm running into some trouble with the GNU linker.

In short, I have one object which has a .yksir_ykpack ELF section, which is being linked with an object which has a .yksir_yktrace ELF section. The desired outcome is a binary with both of these sections.

Here's the object with the .yksir_ykpack section:

readelf --section-headers /home/vext01/research/yk/target/debug/deps/libykpack-2a9122d94a2f837e.rlib 2>/dev/null
...
File: /home/vext01/research/yk/target/debug/deps/libykpack-2a9122d94a2f837e.rlib(ykpack-2a9122d94a2f837e.4kxe4998mamwcrkf.rcgu.o)
There are 6 section headers, starting at offset 0xc25f8:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .strtab           STRTAB           0000000000000000  000c2580
       0000000000000074  0000000000000000           0     0     1
  [ 2] .text             PROGBITS         0000000000000000  00000040
       0000000000000000  0000000000000000  AX       0     0     4
  [ 3] .yksir_ykpack     PROGBITS         0000000000000000  00000040
       00000000000c24f5  0000000000000000           0     0     16
  [ 4] .note.GNU-stack   PROGBITS         0000000000000000  000c2535
       0000000000000000  0000000000000000           0     0     1
  [ 5] .symtab           SYMTAB           0000000000000000  000c2538
       0000000000000048  0000000000000018           1     2     8
...

Note that this object is a Rust rlib, but it's ELF compatible.

Note also that there other "sub-files" in the rlib. I've elided them.

The other object being linked (containing .yksir_yktrace) is a regular .o file.

And since my section names will be unrecogised by the default linker script, I've used section directives (in inline asm). For example, for .yksir_ykpack:

.section .yksir_ykpack, "", @progbits

Now here's our linker command line (sorry it's long -- it is hard to make a minimal example in my setup):

cc \
-Wl,--as-needed \
-Wl,-z,noexecstack \
-m64 \
-L \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.10c166lfq64r2e3y.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.10qcnu2sbqobows0.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.115niq53kx6tizom.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.120g6826u8fiklab.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.14rek8asgh23tnza.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.154hfu34te2ge40t.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.177trhwt854ryxrc.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.18scvl9vj2f5b06v.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.18vw9lln0d5b3qrw.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1b3h401yq8qa8ur5.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1b77e65v6f58yuv6.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1bx0tifep7a0rrg2.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1f45w8utte9cb7vq.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1gu52jpfp6kyy8ga.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1gxs1zbjd9l1x29m.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1jidej0kgblhzsyd.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.1kxugzjco2szwc38.rcgu.o \
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/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.263wkarma1f4il5c.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.26z1aszmxuxamrl6.rcgu.o \
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/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.32ako2nv80o7xnx9.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.336kexx0suto8yk0.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.34a312torkwd02am.rcgu.o \
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/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.r1w6eadga16ihy4.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.s6ldoaew31npivj.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.tmt3fv8lu8oua6k.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.v51zeo4f699m0ti.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.w55t39wk5qjg2bc.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.xp7r6sx9ppyf9t0.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.y8lfemb87vtfeob.rcgu.o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.zwjbkly56hl6j7y.rcgu.o \
-o \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8 \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.3qhbgyy4j5mr4hcm.rcgu.o \
-pie \
-Wl,-zrelro \
-Wl,-znow \
-nodefaultlibs \
-L \
/home/vext01/research/yk/target/debug/deps \
-L \
/home/vext01/research/yk/target/debug/build/hwtracer-928324455c3b5b0d/out \
-L \
/home/vext01/.cargo/git/checkouts/hwtracer-07feaf5fa95de145/fdce391/c_deps/inst/lib \
-L \
/home/vext01/research/yk/target/debug/build/hwtracer-928324455c3b5b0d/out \
-L \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib \
-Wl,-Bstatic \
/home/vext01/research/yk/target/debug/deps/libmemmap-5f23cb5259f61858.rlib \
/home/vext01/research/yk/target/debug/deps/libgimli-d46f6b1d6250e62c.rlib \
/home/vext01/research/yk/target/debug/deps/libindexmap-a7445e071182095b.rlib \
/home/vext01/research/yk/target/debug/deps/libarrayvec-4d4c4abac05eec7a.rlib \
/home/vext01/research/yk/target/debug/deps/libnodrop-3da1b04b3cad85a5.rlib \
/home/vext01/research/yk/target/debug/deps/libstable_deref_trait-7630eaff18b8ab57.rlib \
/home/vext01/research/yk/target/debug/deps/libelf-97ced5ab8064f12b.rlib \
/home/vext01/research/yk/target/debug/deps/libbyteorder-a1ca60651d3abac3.rlib \
/home/vext01/research/yk/target/debug/deps/libphdrs-b9f0bd244ef345ca.rlib \
/home/vext01/research/yk/target/debug/deps/libobject-78dffdc0007e71b6.rlib \
/home/vext01/research/yk/target/debug/deps/libparity_wasm-6e118c1ea26bb0ca.rlib \
/home/vext01/research/yk/target/debug/deps/libflate2-a0dbcb32ab5b92bb.rlib \
/home/vext01/research/yk/target/debug/deps/libminiz_oxide-feee87a1e827d34e.rlib \
/home/vext01/research/yk/target/debug/deps/libadler32-df5c7e6116a31c83.rlib \
/home/vext01/research/yk/target/debug/deps/libcrc32fast-e74dc8ce28b7f377.rlib \
/home/vext01/research/yk/target/debug/deps/libgoblin-31c3e1931784d72b.rlib \
/home/vext01/research/yk/target/debug/deps/liblog-6bd6a1a9417c9b44.rlib \
/home/vext01/research/yk/target/debug/deps/libplain-815d5b934f3c458c.rlib \
/home/vext01/research/yk/target/debug/deps/libscroll-57c304c9c9aa4f19.rlib \
/home/vext01/research/yk/target/debug/deps/libuuid-d9fa09a0ac505087.rlib \
/home/vext01/research/yk/target/debug/deps/libtarget_lexicon-bd6c9c95040de744.rlib \
/home/vext01/research/yk/target/debug/deps/libfailure-883b6313101c519f.rlib \
/home/vext01/research/yk/target/debug/deps/libhwtracer-4a7fcfc6cdb25d08.rlib \
/home/vext01/research/yk/target/debug/deps/libtempfile-f73c3fca6b62eb80.rlib \
/home/vext01/research/yk/target/debug/deps/librand-df0ca11adb03b8e3.rlib \
/home/vext01/research/yk/target/debug/deps/librand_chacha-276a59e9166a9193.rlib \
/home/vext01/research/yk/target/debug/deps/libc2_chacha-170e0ce215266f6e.rlib \
/home/vext01/research/yk/target/debug/deps/libppv_lite86-8dd4ff65271756be.rlib \
/home/vext01/research/yk/target/debug/deps/librand_core-4c118f0513457ff6.rlib \
/home/vext01/research/yk/target/debug/deps/libgetrandom-a724117bf2304247.rlib \
/home/vext01/research/yk/target/debug/deps/libremove_dir_all-635dd99a7c406912.rlib \
/home/vext01/research/yk/target/debug/deps/libcfg_if-6454a07998dfdf77.rlib \
/home/vext01/research/yk/target/debug/deps/libtime-de6100a3b17d5591.rlib \
/home/vext01/research/yk/target/debug/deps/liblibc-7376e8de49ab6c4d.rlib \
/home/vext01/research/yk/target/debug/deps/libykpack-2a9122d94a2f837e.rlib \
/home/vext01/research/yk/target/debug/deps/librmp_serde-337cde3a4d7cf067.rlib \
/home/vext01/research/yk/target/debug/deps/libserde-af0b54eec19c1f21.rlib \
/home/vext01/research/yk/target/debug/deps/librmp-3e2e145986c5b5a2.rlib \
/home/vext01/research/yk/target/debug/deps/libnum_traits-e132ced7a5978735.rlib \
/home/vext01/research/yk/target/debug/deps/libbyteorder-06c02cccfe57c005.rlib \
/home/vext01/research/yk/target/debug/deps/libfallible_iterator-72f65c2f6c00d6f7.rlib \
/home/vext01/research/yk/target/debug/deps/liblazy_static-40baa21fb02fe58c.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libtest-fa7c5db992c603ae.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libterm-677b7a33a112844b.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libgetopts-3823ca10bcc34fef.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunicode_width-b085949ade5be118.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_std-937f89c7b4331592.rlib \
-Wl,--start-group \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-d5a0f5d9c2ecbc13.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-6214af4642f83019.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libhashbrown-d853b38ef82d1b80.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_alloc-b5c8492ce140f2f4.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libbacktrace-4f884e18d6db8201.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libbacktrace_sys-55bf2675880f6c0f.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_demangle-e80dead6761c88af.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-a0079eacbe814aa8.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcfg_if-eaebc19628616584.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-041597c76bfe99c8.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-4f9776276b6dc54c.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_core-f0c8c9c0de0535e4.rlib \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-cf28f55e983f3931.rlib \
-Wl,--end-group \
/home/vext01/research/ykrustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-f4dd3b487c184fe0.rlib \
-Wl,-Bdynamic \
-lipt \
-lutil \
-lutil \
-ldl \
-lrt \
-lpthread \
-lgcc_s \
-lc \
-lm \
-lrt \
-lpthread \
-lutil \
-lutil \
-Wl,--no-gc-sections \
/home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8.2qajgdgxxha7pskm.rcgu.o

This suceeds, but when we inspect the resulting binary, .yksir_ykpack is missing:

$ readelf --section-headers /home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8 2>/dev/null | grep 'yksir_ykpack'
$

The .yksir_yktrace section is in the binary though:

$ readelf --section-headers /home/vext01/research/yk/target/debug/deps/yktrace-191ae338a3a7cbe8 2>/dev/null | grep 'yksir_yktrace'
...
  [31] .yksir_yktrace    PROGBITS         0000000000000000  009ba0e0
       0000000000078ed3  0000000000000000           0     0     16

Does anyone know why this is? Where did .yksir_ykpack go?

I've tried removing -Wl,--as-needed and moving up -Wl,--no-gc-sections. No joy.

Thank you!

Answer 1

I've figured it out.

The crucial detail here is that Rust .rlib files are static libraries. Unlike .o object files, when linking a static library (usually a .a file), the linker uses a resolution step to pull in only as yet undefined symbols and nothing else.

This can be seen using this minimal example:

all: main_a main_o main_a_whole dump

# Generate text files to use as the contents of the sections.
my_section1.raw:
    echo "mydata1" > $@

my_section2.raw:
    echo "mydata2" > $@

# Generate object files for the sections.
my_section1.o: my_section1.raw
    objcopy --input-target binary \
        --output-target=elf64-x86-64 \
        --binary-architecture i386:x86-64 \
        --rename-section .data=.my_section1 \
        my_section1.raw $@

my_section2.o: my_section2.raw
    objcopy --input-target binary \
        --output-target=elf64-x86-64 \
        --binary-architecture i386:x86-64 \
        --rename-section .data=.my_section2 \
        my_section2.raw $@

# And for section1 only, we will make a static lib.
my_section1.a: my_section1.o
    ar rcs $@ my_section1.o

# A "main" stub to link our objects to.
main.c:
    echo "void main(void) {}" > main.c

# Now make some binaries using varying linkage configurations.
# main_o is made from the object files.
main_o: main.o my_section1.a my_section2.o
    ${CC} -o $@ my_section1.o my_section2.o main.o

# main_a uses the static lib for section1 and the object file for section2.
main_a: main.o my_section1.a my_section2.o
    ${CC} -o $@ my_section1.a my_section2.o main.o

# main_a_whole is the same as main_a, but uses --whole-archive when linking section1.
main_a_whole: main.o my_section1.a my_section2.o
    ${CC} -o $@ -Wl,--whole-archive my_section1.a -Wl,--no-whole-archive my_section2.o main.o

# Print the sections of interest from the resulting binaries.
dump:
    @echo "\n>>> main_o sections"
    objdump -h main_o | grep my_section

    @echo "\n>>> main_a sections"
    objdump -h main_a | grep my_section

    @echo "\n>>> main_a_whole sections"
    objdump -h main_a_whole | grep my_section

clean:
    rm -f *.o *.a main_a main_o main_a_whole *.raw main.c

When run, we get:

>>> main_o sections
objdump -h main_o | grep my_section
 23 .my_section1  00000008  0000000000004028  0000000000004028  00003028  2**0
 24 .my_section2  00000008  0000000000004030  0000000000004030  00003030  2**0

>>> main_a sections
objdump -h main_a | grep my_section
 23 .my_section2  00000008  0000000000004028  0000000000004028  00003028  2**0

>>> main_a_whole sections
objdump -h main_a_whole | grep my_section
 23 .my_section1  00000008  0000000000004028  0000000000004028  00003028  2**0
 24 .my_section2  00000008  0000000000004030  0000000000004030  00003030  2**0

Note the use of --whole-archive to force the whole static lib to be loaded. This may be undesirable, as you may wish to fully link only one section, which cannot be done directly AFAICS. In such as case you can extract the section using ar or objcopy and link that instead.