Why GCC compiled C program needs .eh_frame section?

Question

Test is on 32 bit x86 Linux with gcc 4.6.3

When using gcc to compile a C program and using readelf to check the section info, I can see the .eh_frame section and .eh_frame_hdr sections inside.

For example, here is the section info of binary program Perlbench.

readelf -S perlbench

There are 28 section headers, starting at offset 0x102e48:

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        08048154 000154 000013 00   A  0   0  1
[ 2] .note.ABI-tag     NOTE            08048168 000168 000020 00   A  0   0  4
[ 3] .note.gnu.build-i NOTE            08048188 000188 000024 00   A  0   0  4
[ 4] .gnu.hash         GNU_HASH        080481ac 0001ac 000044 04   A  5   0  4
[ 5] .dynsym           DYNSYM          080481f0 0001f0 0007b0 10   A  6   1  4
[ 6] .dynstr           STRTAB          080489a0 0009a0 0003d6 00   A  0   0  1
[ 7] .gnu.version      VERSYM          08048d76 000d76 0000f6 02   A  5   0  2
[ 8] .gnu.version_r    VERNEED         08048e6c 000e6c 0000a0 00   A  6   2  4
[ 9] .rel.dyn          REL             08048f0c 000f0c 000028 08   A  5   0  4
[10] .rel.plt          REL             08048f34 000f34 000388 08   A  5  12  4
[11] .init             PROGBITS        080492bc 0012bc 00002e 00  AX  0   0  4
[12] .plt              PROGBITS        080492f0 0012f0 000720 04  AX  0   0 16
[13] .text             PROGBITS        08049a10 001a10 0cf86c 00  AX  0   0 16
[14] .fini             PROGBITS        0811927c 0d127c 00001a 00  AX  0   0  4
[15] .rodata           PROGBITS        081192a0 0d12a0 017960 00   A  0   0 32
[16] .eh_frame_hdr     PROGBITS        08130c00 0e8c00 003604 00   A  0   0  4
[17] .eh_frame         PROGBITS        08134204 0ec204 01377c 00   A  0   0  4
[18] .ctors            PROGBITS        08148f0c 0fff0c 000008 00  WA  0   0  4
[19] .dtors            PROGBITS        08148f14 0fff14 000008 00  WA  0   0  4
[20] .jcr              PROGBITS        08148f1c 0fff1c 000004 00  WA  0   0  4
[21] .dynamic          DYNAMIC         08148f20 0fff20 0000d0 08  WA  6   0  4
[22] .got              PROGBITS        08148ff0 0ffff0 000004 04  WA  0   0  4
[23] .got.plt          PROGBITS        08148ff4 0ffff4 0001d0 04  WA  0   0  4
[24] .data             PROGBITS        081491e0 1001e0 002b50 00  WA  0   0 32
[25] .bss              NOBITS          0814bd40 102d30 002b60 00  WA  0   0 32
[26] .comment          PROGBITS        00000000 102d30 00002a 01  MS  0   0  1
[27] .shstrtab         STRTAB          00000000 102d5a 0000ec 00      0   0  1

In my understanding, these two sections are used for handling exceptions, it produce tables that describe how to unwind the stack.

But it is for C++ program, they use eh_frame and gcc_exception_table sections to manage exceptions, then why does compiler put the eh_frame and eh_frame_hdr sections inside ELF compiled from C program?

Answer 1

First of all, the original reason for this was largely political - the people who added DWARF-based unwinding (.eh_frame) wanted it to be a feature that's always there so it could be used for implementing all kinds of stuff other than just C++ exceptions, including:

• backtrace()
• __attribute__((__cleanup__(f)))
• __builtin_return_address(n), for n>0
• pthread_cleanup_push, implemented in terms of __attribute__((__cleanup__(f)))
• ...

However if you don't need any of these things, .eh_frame is something like a 15-30% increase to .text size with no benefit. You can disable generation of .eh_frame with -fno-asynchronous-unwind-tables for individual translation units, and this mostly eliminates the size cost, although you still have a few left over coming from crtbegin.o, etc. You cannot strip them with the strip command later; since .eh_frame is a section that lives in the loaded part of the program (this is the whole point), stripping it modifies the binary in ways that break it at runtime. See https://sourceware.org/bugzilla/show_bug.cgi?id=14037 for an example of how things can break.

Note that DWARF tables are also used for debugging, but for this purpose they do not need to be in the loadable part of the program. Using -fno-asynchronous-unwind-tables will not break debugging, because as long as -g is also passed to the compiler, the tables still get generated; they just get stored in a separate, non-loadable, strippable section of the binary, .debug_frame.