RE: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

David Laight
2024-10-06T18:01:12Z

...
> Due to the non-negligible impact of PIE, perhaps some kind of
> CONFIG_PIE config definition should be introduced, so the assembly
> code would be able to choose optimal asm sequence when PIE and non-PIE
> is requested?

I wouldn't have thought that performance mattered in the asm code
that runs during startup?

While x86-84 code (ignoring data references) is pretty much always
position independent, the same isn't true of all architectures.
Some (at least Nios-II) only have absolute call instructions.
So you can't really move to pic code globally.

You'd also want 'bad' pic code that contained some fixups that
needed the code patching.
(Which you really don't want for a shared library.)
Otherwise you get an extra instruction for non-trivial data
accesses.

Thinking....
Doesn't the code generated for -fpic assume that the dynamic loader
has processed the relocations before it is run?
But the kernel startup code is running before they can have been done?
So even if that C code were 'pic' it could still contain things that
are invalid (probably arrays of pointers?).
So you lose one set of bugs and gain another.

David

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