Re: [RFC PATCH 06/10] hurd: Make sure to not use tcb->self

[Joseph Myers]

On Fri, 19 May 2023, Sergey Bugaev via Libc-alpha wrote:

> 'foo' is a public symbol, to be used by the user, and also
> interposable by the user. Always called via PLT (except from inside
> the user's code when redefined inside the executable, in which case
> the compiler/linker will see that no PLT is needed), and should not be
> called from inside glibc.

Should not be called from within glibc via the PLT within the same 
library.

It's OK for foo to be called from bar if foo is part of all the standards 
that contain bar.  But in that case, if the call is from the same library, 
*_hidden_def / *_hidden_proto should be used to avoid calling via the PLT.

If foo is not part of all the standards that contain bar, then glibc code 
for bar should use __foo instead to avoid namespace issues, especially for 
static linking.

If __foo is needed by executables, *_nonshared.a or other shared 
libraries, then it also needs to be exported from the library it's in (in 
which case PLT avoidance is also relevant for __foo, so *_hidden_* should 
be used for __foo).

If __foo is only used within a single library and doesn't need to be 
exported, it's OK to declare it directly with attribute_hidden rather than 
using *_hidden_* to create __GI___foo as an alias.  (In this case, if you 
forget to use attribute_hidden, you won't get test failures - the symbol 
in fact won't get exported, because only symbols explicitly listed in the 
version maps get exported, and so it won't get a PLT entry.  But in some 
cases, code generation is more efficient if the compiler knows that the 
symbol is hidden.  So when you're calling an unexported symbol in the same 
library, it's still desirable for it to be declared as hidden in a way 
visible to the compiler.)  The more complicated hidden_proto / hidden_def 
approach also works for unexported symbols used within a single library, 
it's just more complicated than necessary in that case.

-- 
Joseph S. Myers
joseph@codesourcery.com