Re: [PATCH] Hurd: Enable ifunc by default

[Samuel Thibault via Libc-alpha <libc-alpha@sourceware.org>]

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(leaving gcc out, it's really out of the story, it just happens to
expose support for ifunc)

Joseph Myers, le lun. 18 janv. 2021 20:05:44 +0000, a ecrit:
> /scratch/jmyers/glibc-bot/install/compilers/i686-gnu/lib/gcc/i686-glibc-gnu/11.0.0/../../../../i686-glibc-gnu/bin/ld: /scratch/jmyers/glibc-bot/build/compilers/i686-gnu/glibc/i686-gnu/elf/librtld.os: in function `hurd_file_name_lookup_retry':
> (.text+0x1e08e): undefined reference to `strncpy'

The story seems complex and related to the glibc rtld build rules.

- We need hurd/lookup-retry.c in the rtld
- It happens to use strncpy since ee11682d4f5 ("hurd: Fix strcpy calls")
- strncpy happens to have a hidden def in libc
- hurd/lookup-retry.os thus refers to __GI_strncpy rather than strncpy
- dl-allobjs.os thus includes strncpy-c.os (that provides __GI_strncpy),
and not strncpy.os (that provides strncpy)
- librtld.map thus contains strncpy-c.os only, and not strncpy.os
- thus no rtld-strncpy.os is getting built
- thus strncpy is not available in rtld
- but strncpy doesn't have a hidden def in rtld
- and thus hurd/rtld-lookup-retry.os refers to strncpy, not __GI_strncpy
- thus the link failure.

The base issue I see here is that dl-allobjs is based on libc-built
objects that might be using hidden defs while rtld-built objects might
not.

But we could also say that it's strncpy that should also have a hidden
def in rtld. The attached patch does this, is this ok?

Samuel

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diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index ea126ae70c..051d6b9492 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -856,6 +856,8 @@ for linking")
 # define HIDDEN_BUILTIN_JUMPTARGET(name) HIDDEN_JUMPTARGET(name)
 #endif
 
+#define rtld_hidden_builtin_proto(name, attrs...) rtld_hidden_proto (name, ##attrs)
+
 #if IS_IN (libutil)
 # define libutil_hidden_proto(name, attrs...) hidden_proto (name, ##attrs)
 # define libutil_hidden_tls_proto(name, attrs...) \
diff --git a/include/string.h b/include/string.h
index 81dab39891..27cbde47ef 100644
--- a/include/string.h
+++ b/include/string.h
@@ -144,6 +144,7 @@ libc_hidden_builtin_proto (strcspn)
 libc_hidden_builtin_proto (strlen)
 libc_hidden_builtin_proto (strncmp)
 libc_hidden_builtin_proto (strncpy)
+rtld_hidden_builtin_proto (strncpy)
 libc_hidden_builtin_proto (strpbrk)
 libc_hidden_builtin_proto (stpcpy)
 libc_hidden_builtin_proto (strrchr)