ELF map IP to offset in .text section

You need several pieces of info:

1. The image relocation IR (delta between where the image was loaded and where it was linked to load at).
2. The file offset FO of the LOAD segment containing the instruction and its linked-at address VA.

Then the file offset of instruction at address I is I - IR - VA + FO.

For example, using /usr/bin/date on my system:

readelf -Wl /usr/bin/date

Elf file type is DYN (Position-Independent Executable file)
...
Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  PHDR           0x000040 0x0000000000000040 0x0000000000000040 0x0002d8 0x0002d8 R   0x8
  INTERP         0x000318 0x0000000000000318 0x0000000000000318 0x00001c 0x00001c R   0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x002a28 0x002a28 R   0x1000
  LOAD           0x003000 0x0000000000003000 0x0000000000003000 0x00eb59 0x00eb59 R E 0x1000
  LOAD           0x012000 0x0000000000012000 0x0000000000012000 0x005798 0x005798 R   0x1000
...
 Section to Segment mapping:
  Segment Sections...
   00     
   01     .interp 
   02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt 
   03     .init .plt .plt.got .text .fini 
...

gdb -q /usr/bin/date

(gdb) b clock_gettime
Breakpoint 1 at 0x3100
(gdb) run 
Starting program: /usr/bin/date 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1.1, __GI___clock_gettime (clock_id=0, tp=0x7fffffffd5d0) at ../sysdeps/unix/sysv/linux/clock_gettime.c:38
warning: 38     ../sysdeps/unix/sysv/linux/clock_gettime.c: No such file or directory
(gdb) up
#1  0x0000555555557c65 in ?? ()

(gdb) disas/r 0x555555557c60,0x555555557c67
Dump of assembler code from 0x555555557c60 to 0x555555557c67:
   0x0000555555557c60:  e8 3b 31 00 00          call   0x55555555ada0
=> 0x0000555555557c65:  e9 08 ff ff ff          jmp    0x555555557b72

(gdb) info prog
Last stopped for thread 1 (Thread 0x7ffff7dae740 (LWP 3767408)).
        Using the running image of child process 3767408.
Program stopped at 0x7ffff7e8be40.
It stopped at breakpoint 1.
Type "info stack" or "info registers" for more information.
(gdb) shell grep /usr/bin/date /proc/3767408/maps
555555554000-555555557000 r--p 00000000 fd:01 115609125                  /usr/bin/date
555555557000-555555566000 r-xp 00003000 fd:01 115609125                  /usr/bin/date
555555566000-55555556c000 r--p 00012000 fd:01 115609125                  /usr/bin/date
55555556c000-55555556d000 r--p 00018000 fd:01 115609125                  /usr/bin/date
55555556d000-55555556e000 rw-p 00019000 fd:01 115609125                  /usr/bin/date

We expect to find e8 3b 31 00 00 e9 08 ff ff ff bytes at file offset 0x0000555555557c60 - 0x555555554000 - 0x003000 + 0x003000 == 0x3c60.

xxd -g1 /usr/bin/date | grep 003c60:
00003c60: e8 3b 31 00 00 e9 08 ff ff ff 80 7c 24 3f 00 0f  .;1........|$?..

QED.