[PATCH] libdwfl: use GNU strerror_r only when available.

[PATCH] libdwfl: use GNU strerror_r only when available.

[Érico Nogueira]

From: Érico Rolim <erico.erc@gmail.com>

Some C libraries don't provide the GNU version of strerror_r, only the
XSI-compliant one. We use the GNU version when available, since it fits
the code better, and otherwise use the XSI-compliant one.

Signed-off-by: Érico Rolim <erico.erc@gmail.com>
---

If possible, I'd like to get this patch in before the release, otherwise
it's an easy one to carry locally.

 ChangeLog            |  4 ++++
 configure.ac         | 12 ++++++++++++
 libdwfl/ChangeLog    |  4 ++++
 libdwfl/dwfl_error.c | 17 ++++++++++++++++-
 4 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 142caa27..c8f921a5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2021-02-01  Érico Nogueira  <ericonr@disroot.org>
+
+	* configure.ac: Check for GNU strerror_r.
+
 2021-01-12  Dmitry V. Levin  <ldv@altlinux.org>
 
 	* configure.ac [--enable-gcov]: Check for gcov, lcov, and genhtml.
diff --git a/configure.ac b/configure.ac
index 346ab800..baf6faf5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -428,6 +428,18 @@ AC_CHECK_DECLS([mempcpy],[],[],
 
 AC_CHECK_FUNCS([process_vm_readv])
 
+AC_CACHE_CHECK([whether C library provides GNU strerror_r], ac_cv_gnu_strerror_r, [dnl
+old_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([dnl
+#define _GNU_SOURCE
+#include <string.h>
+char * (*s)(int, char*, size_t) = strerror_r;
+])], ac_cv_gnu_strerror_r=yes, ac_cv_gnu_strerror_r=no)
+CFLAGS="$old_CFLAGS"])
+AS_IF([test "x$ac_cv_gnu_strerror_r" = "xyes"],
+      [AC_DEFINE([HAVE_GNU_STRERROR_R], [1], [Defined if libc has GNU style strerror_r])])
+
 AC_CHECK_LIB([stdc++], [__cxa_demangle], [dnl
 AC_DEFINE([USE_DEMANGLE], [1], [Defined if demangling is enabled])])
 AM_CONDITIONAL(DEMANGLE, test "x$ac_cv_lib_stdcpp___cxa_demangle" = "xyes")
diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 5058f5f8..d107e78f 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,7 @@
+2021-02-01  Érico Nogueira  <ericonr@disroot.org>
+
+	* dwfl_error.c (strerror_r): Only use the GNU version when available.
+
 2021-01-08  Timm Bäder  <tbaeder@redhat.com>
 
 	* elf-from-memory.c (elf_from_remote_memory): Add for loop over
diff --git a/libdwfl/dwfl_error.c b/libdwfl/dwfl_error.c
index 7bcf61cc..ef2834b2 100644
--- a/libdwfl/dwfl_error.c
+++ b/libdwfl/dwfl_error.c
@@ -137,6 +137,21 @@ __libdwfl_seterrno (Dwfl_Error error)
 }
 
 
+static const char *
+errnomsg(int error)
+{
+  /* Won't be changed by strerror_r, but not const so compiler doesn't throw warning */
+  static char unknown[] = "unknown error";
+
+#ifdef HAVE_GNU_STRERROR_R
+  return strerror_r (error, unknown, 0);
+#else
+  /* To store the error message from strerror_r in a thread-safe manner */
+  static __thread char msg[128];
+  return strerror_r (error, msg, sizeof (msg)) ? unknown : msg;
+#endif
+}
+
 const char *
 dwfl_errmsg (int error)
 {
@@ -154,7 +169,7 @@ dwfl_errmsg (int error)
   switch (error &~ 0xffff)
     {
     case OTHER_ERROR (ERRNO):
-      return strerror_r (error & 0xffff, "bad", 0);
+      return errnomsg (error & 0xffff);
     case OTHER_ERROR (LIBELF):
       return elf_errmsg (error & 0xffff);
     case OTHER_ERROR (LIBDW):
-- 
2.30.0

Re: [PATCH] libdwfl: use GNU strerror_r only when available.

[Mark Wielaard]

Hi Érico,

On Mon, Feb 01, 2021 at 03:56:26PM -0300, Érico Nogueira via Elfutils-devel wrote:
> Some C libraries don't provide the GNU version of strerror_r, only the
> XSI-compliant one. We use the GNU version when available, since it fits
> the code better, and otherwise use the XSI-compliant one.

Could you also mention this bug in the commit message?
https://sourceware.org/bugzilla/show_bug.cgi?id=21010

> If possible, I'd like to get this patch in before the release, otherwise
> it's an easy one to carry locally.

One other comment, but this looks good to go otherwise.

> diff --git a/configure.ac b/configure.ac
> index 346ab800..baf6faf5 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -428,6 +428,18 @@ AC_CHECK_DECLS([mempcpy],[],[],
>  
>  AC_CHECK_FUNCS([process_vm_readv])
>  
> +AC_CACHE_CHECK([whether C library provides GNU strerror_r], ac_cv_gnu_strerror_r, [dnl
> +old_CFLAGS="$CFLAGS"
> +CFLAGS="$CFLAGS -Werror"
> +AC_COMPILE_IFELSE([AC_LANG_SOURCE([dnl
> +#define _GNU_SOURCE
> +#include <string.h>
> +char * (*s)(int, char*, size_t) = strerror_r;
> +])], ac_cv_gnu_strerror_r=yes, ac_cv_gnu_strerror_r=no)
> +CFLAGS="$old_CFLAGS"])
> +AS_IF([test "x$ac_cv_gnu_strerror_r" = "xyes"],
> +      [AC_DEFINE([HAVE_GNU_STRERROR_R], [1], [Defined if libc has GNU style strerror_r])])
> +

autoconf comes with the following check:

 - Macro: AC_FUNC_STRERROR_R

    If strerror_r is available, define HAVE_STRERROR_R, and if it is
    declared, define HAVE_DECL_STRERROR_R. If it returns a char *
    message, define STRERROR_R_CHAR_P; otherwise it returns an int
    error number. The Thread-Safe Functions option of Posix requires
    strerror_r to return int, but many systems (including, for
    example, version 2.2.4 of the GNU C Library) return a char * value
    that is not necessarily equal to the buffer argument.

Can we use that instead?

> +static const char *
> +errnomsg(int error)
> +{
> +  /* Won't be changed by strerror_r, but not const so compiler doesn't throw warning */
> +  static char unknown[] = "unknown error";
> +
> +#ifdef HAVE_GNU_STRERROR_R

And use #ifdef STRERROR_R_CHAR_P here?

Thanks,

Mark

Re: [PATCH] libdwfl: use GNU strerror_r only when available.

[Érico Nogueira]

Em 01/02/2021 18:35, Mark Wielaard escreveu:
> Hi Érico,
> 
> On Mon, Feb 01, 2021 at 03:56:26PM -0300, Érico Nogueira via Elfutils-devel wrote:
>> Some C libraries don't provide the GNU version of strerror_r, only the
>> XSI-compliant one. We use the GNU version when available, since it fits
>> the code better, and otherwise use the XSI-compliant one.
> 
> Could you also mention this bug in the commit message?
> https://sourceware.org/bugzilla/show_bug.cgi?id=21010

Will do!

> 
>> If possible, I'd like to get this patch in before the release, otherwise
>> it's an easy one to carry locally.
> 
> One other comment, but this looks good to go otherwise.
> 
>> diff --git a/configure.ac b/configure.ac
>> index 346ab800..baf6faf5 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -428,6 +428,18 @@ AC_CHECK_DECLS([mempcpy],[],[],
>>   
>>   AC_CHECK_FUNCS([process_vm_readv])
>>   
>> +AC_CACHE_CHECK([whether C library provides GNU strerror_r], ac_cv_gnu_strerror_r, [dnl
>> +old_CFLAGS="$CFLAGS"
>> +CFLAGS="$CFLAGS -Werror"
>> +AC_COMPILE_IFELSE([AC_LANG_SOURCE([dnl
>> +#define _GNU_SOURCE
>> +#include <string.h>
>> +char * (*s)(int, char*, size_t) = strerror_r;
>> +])], ac_cv_gnu_strerror_r=yes, ac_cv_gnu_strerror_r=no)
>> +CFLAGS="$old_CFLAGS"])
>> +AS_IF([test "x$ac_cv_gnu_strerror_r" = "xyes"],
>> +      [AC_DEFINE([HAVE_GNU_STRERROR_R], [1], [Defined if libc has GNU style strerror_r])])
>> +
> 
> autoconf comes with the following check:
> 
>   - Macro: AC_FUNC_STRERROR_R
> 
>      If strerror_r is available, define HAVE_STRERROR_R, and if it is
>      declared, define HAVE_DECL_STRERROR_R. If it returns a char *
>      message, define STRERROR_R_CHAR_P; otherwise it returns an int
>      error number. The Thread-Safe Functions option of Posix requires
>      strerror_r to return int, but many systems (including, for
>      example, version 2.2.4 of the GNU C Library) return a char * value
>      that is not necessarily equal to the buffer argument.
> 
> Can we use that instead?

Thank you for pointing me at it :)

I was a bit worried about rolling my own check here, glad to find out 
there is a standard way of doing it.

> 
>> +static const char *
>> +errnomsg(int error)
>> +{
>> +  /* Won't be changed by strerror_r, but not const so compiler doesn't throw warning */
>> +  static char unknown[] = "unknown error";
>> +
>> +#ifdef HAVE_GNU_STRERROR_R
> 
> And use #ifdef STRERROR_R_CHAR_P here?
> 
> Thanks,
> 
> Mark
>