ELF Format notes (7) -- symbol table, elfformat

ELF Format notes (7) -- symbol table, elfformat

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The symbol table contains the definition or reference information of the symbol. For the android so file, the. dynsym symbol table stores the Import and Export symbols of the library file.

Read the. dynsym dynamic symbol table of android liblog. so with readelf:

 

The Ndx (st_shndx) of the symbol _ cxa_finalize is UND, indicating that the symbol is not defined in this so and needs to be searched in the liblog. so dependency library. Which libraries does liblog. so depend on? It can be found in. dynamic.

 

The definition of the symbol _ cxa_finalize should be in libc. so.

The Ndx (st_shndx) of the symbol _ android_log_vprint is 8, indicating that the symbol is defined in this so, and the section defining the symbol has an index of 8 in the section header table. Take a look at section header table:

 

We can see that the symbol _ android_log_vprint is defined in. text section, which is also taken for granted. Therefore, _ android_log_vprint is an export symbol, which is called by programs that need to print logs.

Because the symbol _ android_log_vprint is defined in this so, the Value (st_value) is not 0. Android linker finds the address defined by the symbol in the memory by the st_value when performing symbol relocation. However, 0x00001539 is of course not the final memory address of the symbol. linker will add the base address of liblog. so in the memory, so st_value is equivalent to an address offset.

Looking back, the symbol table is actually a struct array, and each struct describes the information of a symbol. Struct is defined as follows:

 

St_value and st_shndx are described above.

St_name is the name of the symbol. Here, only an index value that references the string table is saved.

St_size is 0 for the import symbol because it does not know the size of the person's home. The size of the exported symbol is clearly indicated.

St_info contains information such as the symbol type and binding attribute, which will be detailed in subsequent notes.

St_other is retained and does not need to be concerned.