The other day I found a really slick way of adding arbitrary data to an .o (object) file with ‘ld’. It is probably easiest to show the setup, then explain it.<\/p>\n
test.data<\/strong> extra.h<\/strong> extra.c<\/strong> main.c<\/strong> void print_str(const char *ptr, int length);<\/p>\n int main() Compilation Steps<\/strong> note: print_str prints character by character to a specified length LD creates a new ELF segment within the extra.o object file. readelf and objdump will give quite a bit of useful information regarding the segments that will eventually get linked into the executable <\/p>\n # objdump -t extra.o<\/strong><\/p>\n The start of the test.data file will be located in the variable ‘_binary_test_data_start’ and similary the end will be ‘_binary_test_data_end’.<\/p>\n This might be useful for embedding cryptographic signatures, or maybe game data files.<\/p>\n","protected":false},"excerpt":{"rendered":" The other day I found a really slick way of adding arbitrary data to an .o (object) file with ‘ld’. It is probably easiest to show the setup, then explain it. test.data Hello, World extra.h extern const char _binary_test_data_start[]; extern const char _binary_test_data_end[]; extra.c #include “extra.h” \/* this C file when compiled to an object … Continue reading Very Late Assignments with LD<\/span><\/a><\/p>\n","protected":false},"author":21,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[3],"tags":[],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/posts\/7"}],"collection":[{"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/comments?post=7"}],"version-history":[{"count":1,"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/posts\/7\/revisions"}],"predecessor-version":[{"id":14,"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/posts\/7\/revisions\/14"}],"wp:attachment":[{"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/media?parent=7"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/categories?post=7"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.gentoo.org\/rphillips\/wp-json\/wp\/v2\/tags?post=7"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
\n
\nHello, World
\n<\/code><\/p>\n
\n
\nextern const char _binary_test_data_start[];
\nextern const char _binary_test_data_end[];
\n<\/code><\/p>\n
\n
\n#include \"extra.h\"
\n\/* this C file when compiled to an object will include the data found in test.data *\/
\n<\/code><\/p>\n
\n
\n#include \"extra.h\"<\/p>\n
\n{
\n int size = _binary_test_data_end - _binary_test_data_start;
\n print_str(_binary_test_data_start, size);
\n return 0;
\n}
\n<\/code><\/p>\n
\n
\n# gcc extra.c -c
\n# ld -b binary -r -o extra.o test.data
\n# gcc extra.o main.c -o out
\n<\/code><\/p>\n
\nnote2: _binary_test_data_size is exported, but I haven’t been able to figure out how to use it yet.<\/p>\n\r\nextra.o: file format elf32-i386\r\n\r\nSYMBOL TABLE:\r\n00000000 l d .data\t00000000 \r\n00000000 l d *ABS*\t00000000 \r\n00000000 l d *ABS*\t00000000 \r\n00000000 l d *ABS*\t00000000 \r\n0000000c g .data\t00000000 _binary_test_data_end\r\n0000000c g *ABS*\t00000000 _binary_test_data_size\r\n00000000 g .data\t00000000 _binary_test_data_start\r\n<\/pre>\n