Re: STV_PROTECTED

Mike Frysinger
2016-03-09T05:01:36Z

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On 09 Mar 2016 02:03, Michael Kerrisk (man-pages) wrote:
> On 09/29/2015 12:14 PM, Gabriel Corona wrote:
> > The elf manpage [2] claims that:
> >
> >> STV_PROTECTED Not preemptible, not exported.
> >
> > However the System V gABI [1] claims that:
> >
> >> A symbol defined in the current component is protected if it is
> >> visible in other components but not preemptable
> >
> > Should the manpage read something like:
> >
> >> STV_PROTECTED Not preemptible but exported.
> >
> > [1] http://www.sco.com/developers/gabi/2003-12-17/ch4.symtab.html
> >
> > [2] http://man7.org/linux/man-pages/man5/elf.5.html
>
> I think you are right, but let's see if Mike might confirm,
> since he originally added that text. (Mike, I suspect there
> was a wordo here.)

i didn't really author that content, but take it from elf.h in glibc:
#define STV_PROTECTED 3 /* Not preemptible, not exported */

and binutils says:
#define STV_PROTECTED 3 /* Treat as STB_LOCAL inside current component */

and llvm says:
STV_PROTECTED = 3 // Visible in other components but not preemptable

gabi says:
https://refspecs.linuxbase.org/elf/gabi4+/ch4.symtab.html
A symbol defined in the current component is protected if it is visible in other
components but not preemptable, meaning that any reference to such a symbol from
within the defining component must be resolved to the definition in that
component, even if there is a definition in another component that would preempt
by the default rules. A symbol with STB_LOCAL binding may not have STV_PROTECTED
visibility. If a symbol definition with STV_PROTECTED visibility from a shared
object is taken as resolving a reference from an executable or another shared
object, the SHN_UNDEF symbol table entry created has STV_DEFAULT visibility.

solaris/oracle says:
https://docs.oracle.com/cd/E26502_01/html/E26507/chapter6-79797.html
A symbol that is defined in the current component is protected if the symbol is
visible in other components, but cannot be preempted. Any reference to such a
symbol from within the defining component must be resolved to the definition in
that component. This resolution must occur, even if a symbol definition exists
in another component that would interpose by the default rules. A symbol with
STB_LOCAL binding will not have STV_PROTECTED visibility.

but i think this ibm article is probably the most understandable:
https://www.ibm.com/developerworks/aix/library/au-aix-symbol-visibility/
The symbol is visible outside the current executable or shared object, but it
may not be overridden. In other words, if a protected symbol in a shared library
is referenced by an other code in the shared library, the other code will always
reference the symbol in the shared library, even if the executable defines a
symbol with the same name.

so while "Not preemptible but exported" is more correct, i'm not sure it's
still all that great. maybe "Exported; not preemptible in current module." ?
-mike

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