Add basic support for ELF thread local storage segments

[phil-opp]

This reports the thread local storage segment of the kernel (if present) in the boot information. This should make it possible to use the unstable #[thread_local] attribute. Since the TLS template overlaps with parts of a LOAD segment, no additional mapping needs to be done.

Note that the TLS template should not be used directly. Instead, the kernel should create a copy the TLS template for each thread and then set up the fs register accordingly. See https://uclibc.org/docs/tls.pdf for more information.

This code only works for a single TLS segment. If more than one TLS segment are present in the executable, the bootloader errors. We can of course lift this restriction if this becomes a problem in practice.

Fixes #95

[phil-opp]

@vinaychandra Could you try whether this change works for you?

[phil-opp]

Ah, it seems like I understood something wrong. I will create a new version.

[phil-opp]

With the new version, I can now successfully read the TLS template info for a small example:

#[thread_local]
pub static mut DATA: u64 = 0;
#[thread_local]
pub static BSS: u64 = 0;

fn kernel_main(boot_info: &'static BootInfo) -> ! {
    println!("Hello World{}", "!");
    println!("TLS INFO: {:x?}", boot_info.tls_template());

    // this panics because we did not set up the fs register yet
    println!("DATA: {}, BSS: {}", unsafe { DATA }, BSS);
}

The result is the following:

I get the same values with readelf -l target/x86_64-blog_os/debug/blog_os:

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000200040 0x0000000000200040
                 0x00000000000001c0 0x00000000000001c0  R      0x8
  LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                 0x0000000000003544 0x0000000000003544  R      0x1000
  LOAD           0x0000000000004000 0x0000000000204000 0x0000000000204000
                 0x000000000001f23c 0x000000000001f23c  R E    0x1000
  LOAD           0x0000000000024000 0x0000000000224000 0x0000000000224000
                 0x00000000000012d0 0x00000000000012d0  RW     0x1000
  LOAD           0x0000000000026000 0x0000000000226000 0x0000000000226000
                 0x000000000000000e 0x0000000000002178  RW     0x1000
  TLS            0x0000000000024000 0x0000000000224000 0x0000000000224000
                 0x0000000000000008 0x0000000000000010  R      0x8
  GNU_RELRO      0x0000000000024000 0x0000000000224000 0x0000000000224000
                 0x00000000000012d0 0x0000000000002000  R      0x1
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x0

 Section to Segment mapping:
  Segment Sections...
   00     
   01     .rodata .eh_frame 
   02     .text 
   03     .tdata .data.rel.ro .got 
   04     .data .bss 
   05     .tdata .tbss 
   06     .tdata .data.rel.ro .got 
   07     

[phil-opp]

Published as version 0.8.8