How can the offset of a segment Off not divisible by the alignment Al in an ELF file?

Question

[Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
[ 1] .text             PROGBITS        00000000 000034 00002a 00  AX  0   0  4

As above,the segment begin from 0x34 address, but its Al is 4,so it can't be divided by 2**4.

I mean : 0x34 % 16 != 0.So I want to ask why .text segment's address doesn't begin from Integer times of 16.

Answer 1

The section header struct looks like this:

typedef struct {
   uint32_t   sh_name;
   uint32_t   sh_type;
   uint32_t   sh_flags;
   Elf32_Addr sh_addr;
   Elf32_Off  sh_offset;
   uint32_t   sh_size;
   uint32_t   sh_link;
   uint32_t   sh_info;
   uint32_t   sh_addralign;
   uint32_t   sh_entsize;
} Elf32_Shdr;

So what you see under the Al column is sh_addralign . Let's look at the description of that member from the elf manpage :

sh_addralign
             Some sections have address alignment constraints.  If a
             section holds a doubleword, the system must ensure
             doubleword alignment for the entire section.  That is, the
             value of sh_addr must be congruent to zero, modulo the
             value of sh_addralign.  Only zero and positive integral
             powers of two are allowed.  Values of zero or one mean the
             section has no alignment constraints.

TL;DR : The alignment constraint shown in the Al column is for Addr (which is aligned in your case since it's zero), not for Off . In other words, it's an alignment constraint for the address where the image is loaded in memory, not for where it's stored in the ELF file.