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Date: Sat, 8 Jan 2022 20:49:17 -0500
From: Rich Felker <dalias@...c.org>
To: Rui Ueyama <rui314@...il.com>, musl@...ts.openwall.com
Subject: Re: [PATCH] Explicitly pass the -fno-common flag
On Sun, Jan 09, 2022 at 12:25:36AM +0100, Szabolcs Nagy wrote:
> * Rui Ueyama <rui314@...il.com> [2022-01-08 16:45:51 +0900]:
> > Common symbol is a special type of symbol that allows a linker to merge
> > multiple common symbols into one and turn it into a defined symbol.
> > This feature was used in C to allow tentative definitions. That is,
> > you can write `int foo;` instead of `extern int foo;` in a header file.
> >
> > Common symbols are discouraged these days because they can easily
> > hide unintentional duplicate symbol errors. For that reason, GCC
> > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85678)
> > and Clang (https://github.com/llvm/llvm-project/commit/0a9fc9233e172601e26381810d093e02ef410f65)
> > now default to `-fno-common`.
> >
> > That means, musl libc's global variables are compiled to common symbols
> > or regular defined symbols depending on the compiler. That's not an issue
> > per se, but it's unnecessary churn.
> >
> > This patch always passes the `-fno-common` flag to the compiler.
>
> building x86_64 musl with -fcommon i see these symbols in COM section:
>
> obj/src/malloc/replaced.lo:
> __aligned_alloc_replaced
> __malloc_replaced
> obj/src/malloc/mallocng/malloc.lo:
> __malloc_lock
> obj/src/misc/getopt.lo:
> __optpos
> optarg
> optopt
> obj/src/env/__stack_chk_fail.lo:
> __stack_chk_guard
> obj/src/env/__init_tls.lo:
> __thread_list_lock
> obj/src/aio/aio.lo:
> __aio_fut
> obj/src/locale/locale_map.lo:
> __locale_lock
> obj/src/internal/libc.lo:
> __hwcap
> __libc
> obj/src/internal/defsysinfo.lo:
> __sysinfo
> obj/src/signal/sigaction.lo:
> __eintr_valid_flag
> obj/src/exit/abort_lock.lo:
> __abort_lock
> obj/src/network/h_errno.lo:
> h_errno
> obj/src/time/getdate.lo:
> getdate_err
>
> i'm not sure how this can cause trouble (maybe static linking?), but
> if we care then another solution is to change
>
> int x;
>
> to
>
> int x = 0;
>
> instead of forcing -fno-common.
I don't really see a need for us to care whether these are commons or
bss. Having them be bss would give slightly better ability to error on
build-time UB in static linked programs, but doesn't do anything at
all for dynamic linking, and for static linking the problem is only
diagnosable when the data object is in a file that's pulled in for an
undefined symbol other than that of the data object (e.g. a function
or other data object in the same TU).
The one time bss is significantly preferable to common is for
zero-initialized const objects (rare but they might appear in some
places) since commons don't actually get made const. I don't think we
have any of those that are commons now though.
Rich
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