How to reduce ELF section padding?

Question

I used crosstool-NG to create a PowerPC toolchain with gcc 6.3.0 and glibc 2.25. I have the following test program, test.c:

int main() { return 0; }

I compiled it with the command:

powerpc-unknown-linux-gnu-gcc -s -Os -o test test.c

The final binary is 66904 bytes, which is much larger than expected. The section headers look like this:

$ powerpc-unknown-linux-gnu-readelf -S test
There are 27 section headers, starting at offset 0x10120:

Section Headers:
[Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
[ 0]                   NULL            00000000 000000 000000 00      0   0  0
[ 1] .interp           PROGBITS        10000154 000154 00000d 00   A  0   0  1
[ 2] .note.ABI-tag     NOTE            10000164 000164 000020 00   A  0   0  4
[ 3] .hash             HASH            10000184 000184 000024 04   A  4   0  4
[ 4] .dynsym           DYNSYM          100001a8 0001a8 000040 10   A  5   1  4
[ 5] .dynstr           STRTAB          100001e8 0001e8 000045 00   A  0   0  1
[ 6] .gnu.version      VERSYM          1000022e 00022e 000008 02   A  4   0  2
[ 7] .gnu.version_r    VERNEED         10000238 000238 000020 00   A  5   1  4
[ 8] .rela.dyn         RELA            10000258 000258 00000c 0c   A  4   0  4
[ 9] .rela.plt         RELA            10000264 000264 000018 0c  AI  4  23  4
[10] .init             PROGBITS        1000027c 00027c 00004c 00  AX  0   0  4
[11] .text             PROGBITS        100002c8 0002c8 00031c 00  AX  0   0  4
[12] .fini             PROGBITS        100005e4 0005e4 000030 00  AX  0   0  4
[13] .rodata           PROGBITS        10000614 000614 000014 00   A  0   0  4
[14] .eh_frame_hdr     PROGBITS        10000628 000628 000014 00   A  0   0  4
[15] .eh_frame         PROGBITS        1000063c 00063c 000080 00   A  0   0  4
[16] .ctors            PROGBITS        1001ff1c 00ff1c 000008 00  WA  0   0  4
[17] .dtors            PROGBITS        1001ff24 00ff24 000008 00  WA  0   0  4
[18] .jcr              PROGBITS        1001ff2c 00ff2c 000004 00  WA  0   0  4
[19] .got2             PROGBITS        1001ff30 00ff30 000008 00  WA  0   0  1
[20] .dynamic          DYNAMIC         1001ff38 00ff38 0000c8 08  WA  5   0  4
[21] .data             PROGBITS        10020000 010000 000008 00  WA  0   0  4
[22] .got              PROGBITS        10020008 010008 000014 04 WAX  0   0  4
[23] .plt              NOBITS          1002001c 01001c 000060 00 WAX  0   0  4
[24] .bss              NOBITS          1002007c 01001c 000008 00  WA  0   0  4
[25] .comment          PROGBITS        00000000 01001c 00002e 01  MS  0   0  1
[26] .shstrtab         STRTAB          00000000 01004a 0000d4 00      0   0  1

You can see that there's a large jump between the .eh_frame and .ctors sections. If I use 'hd' to look at the file, I can see that the space between .eh_frame and .ctors is filled entirely with null bytes.

Why is gcc adding such a huge amount of space between the sections and is there any way to change its behavior?

Answer 1

It looks like this is due to binutils 2.27 increasing the default page size of PowerPC targets to 64k, resulting in bloated binaries on embedded platforms.

There's a discussion on the crosstool-NG github here.

Configuring binutils with --disable-relro should improve things.

You can also add -Wl,-z,max-page-size=0x1000 to gcc when compiling.