x86: General Dynamic (GD)

This code sequence is the most general, and can be included both in a shared objects and dynamic executables. This code sequence can reference an external TLS variable in either a shared object or dynamic executable.

0x00 leal  x@tlsgd(,%ebx,1),%eax
0x07 call  x@tlsgdplt  

# %eax - contains address of TLS variable

R_386_TLS_GD
R_386_TLS_GD_PLT

x
x

GOT[n]
GOT[n + 1]

R_386_TLS_DTPMOD32
R_386_TLS_DTPOFF32

x

The leal instruction generates a R_386_TLS_GD relocation which instructs the link-editor to allocate space in the global offset table to hold a TLS_index structure for variable x. The link-editor processes this relocation by substituting the GOT-relative offset for the new GOT entry.

Since the load object index and TLS block index for x are not known until runtime, the link-editor places the R_386_TLS_DTPMOD32 and R_386_TLS_DTPOFF32 relocations against the GOT for processing by the runtime linker. The address of the generated GOT entry is loaded into register %eax for the call to ___tls_get_addr().

The call instruction causes the generation of the R_386_TLS_GD_PLT relocation. This instructs the link-editor to bind the call to the ___tls_get_addr() function and associates the call instruction with the GD code sequence.

The call instruction must immediately follow the leal instruction. This is a required to permit the code transformations.