RFC: Audit external function called indirectly via GOT

["H.J. Lu" <hjl.tools@gmail.com>]

Auditing of external function calls and their return values relies on
lazy binding with PLT.  When external functions are called indirectly
via GOT without using PLT, auditing stops working.

Here is a proposal to support auditing of external function called
indirectly via GOT:

1. Add optional dynamic tags:

 #define DT_GNU_PLT     0x6ffffef4  /* Address of PLT section  */
 #define DT_GNU_PLTSZ   0x6ffffdf1  /* Size of PLT section  */
 #define DT_GNU_PLTENT  0x6ffffdf2  /* Size of one PLT entry  */
 #define DT_GNU_PLT0SZ  0x6ffffdf3  /* Size of the first PLT entry  */
 #define DT_GNU_PLTGOTSZ 0x6ffffdf4 /* Size of PLTGOT section  */

and update DT_FLAGS_1 with:

 #define DF_1_JMPRELIGN 0x10000000  /* DT_JMPREL can be ignored  */
2. Linker creates PLT entries for auditing external function calls via
GOT and sets DT_GNU_PLT, DT_GNU_PLTSZ, DT_GNU_PLTENT, DT_GNU_PLT0SZ and
DT_GNU_PLTGOTSZ.  If PLT isn't required for lazy binding, set the
DF_1_JMPRELIGN bit in DT_FLAGS_1.
3. When auditing is enabled at run-time, dynamic linker resolves GLOB_DAT
relocation to its corresponding PLT entry by finding JUMP_SLOT relocation
against the same function and use its PLT slot as the function address.
On x86, the first PLT entry and the 3 GOT slots are reserved.  GOT slot
is (JUMP_SLOT relocation offset - DT_PLTGOT) / size of GOT entry.  PLT
offset is (GOT slot - 3) * DT_GNU_PLTENT + DT_GNU_PLT0SZ.  PLT address
is DT_GNU_PLT + PLT offset.  DT_GNU_PLT, DT_GNU_PLTSZ, DT_PLTGOT and
DT_GNU_PLTGOTSZ can be used to check if GOT and PLT offsets are within
range.
4. If DF_1_JMPRELIGN is set, dynamic linker can ignore DT_JMPREL when
lazy binding is disabled.

Any comments?

H.J.