ldconfig.8: Document file filter and symlink pattern expectations

Message ID xna78awjsl.fsf@greed.delorie.com
State New
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  • ldconfig.8: Document file filter and symlink pattern expectations
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Commit Message

DJ Delorie Dec. 2, 2019, 8:32 p.m.
Information gleaned from comments in glibc's elf/ldconfig.c

Comments

Carlos O'Donell Dec. 2, 2019, 9:41 p.m. | #1
On 12/2/19 3:32 PM, DJ Delorie wrote:
> 

> Information gleaned from comments in glibc's elf/ldconfig.c

> 

> diff --git a/man8/ldconfig.8 b/man8/ldconfig.8

> index 4f799962c..15585243c 100644

> --- a/man8/ldconfig.8

> +++ b/man8/ldconfig.8

> @@ -93,6 +93,28 @@ option.

>  .B ldconfig

>  should normally be run by the superuser as it may require write

>  permission on some root owned directories and files.

> +.PP

> +Note that

> +.B ldconfig

> +will only look at files that are named

> +.I lib*.so*

> +(for regular shared objects) or

> +.I ld-*.so*

> +(for the dynamic loader itsef).  Other files will be ignored.  Also,

> +.B ldconfig

> +expects a certain pattern to how the symlinks are set up, like this

> +example, where the middle file

> +.RB ( libfoo.so.1

> +here) is the SONAME for the library:

> +.PP

> +.in +4n

> +.EX

> +libfoo.so -> libfoo.so.1 -> libfoo.so.1.12

> +.EE

> +.in

> +.PP

> +Failure to follow this pattern may result in compatibility issues

> +after an upgrade.

>  .SH OPTIONS

>  .TP

>  .BR \-c " \fIfmt\fP, " \-\-format=\fIfmt\fP

> 


I wrote the relevant comment in glibc here after a frustrating
night of debugging :-)

elf/ldconfig.c

 884           /* If the path the link points to isn't its soname or it is not
 885              the .so symlink for ld(1), we treat it as a normal file.
 886 
 887              You should always do this:
 888 
 889                 libfoo.so -> SONAME -> Arbitrary package-chosen name.
 890 
 891              e.g. libfoo.so -> libfoo.so.1 -> libfooimp.so.9.99.
 892              Given a SONAME of libfoo.so.1.
 893 
 894              You should *never* do this:
 895 
 896                 libfoo.so -> libfooimp.so.9.99
 897 
 898              If you do, and your SONAME is libfoo.so.1, then libfoo.so
 899              fails to point at the SONAME. In that case ldconfig may consider
 900              libfoo.so as another implementation of SONAME and will create
 901              symlinks against it causing problems when you try to upgrade
 902              or downgrade. The problems will arise because ldconfig will,
 903              depending on directory ordering, creat symlinks against libfoo.so
 904              e.g. libfoo.so.1.2 -> libfoo.so, but when libfoo.so is removed
 905              (typically by the removal of a development pacakge not required
 906              for the runtime) it will break the libfoo.so.1.2 symlink and the
 907              application will fail to start.  */

Should we be more specific about always doing:

	libfoo.so -> SONAME -> Arbitrary package-chosen name.

Users love having examples of how to do it right :-)

-- 
Cheers,
Carlos.

Patch

diff --git a/man8/ldconfig.8 b/man8/ldconfig.8
index 4f799962c..15585243c 100644
--- a/man8/ldconfig.8
+++ b/man8/ldconfig.8
@@ -93,6 +93,28 @@  option.
 .B ldconfig
 should normally be run by the superuser as it may require write
 permission on some root owned directories and files.
+.PP
+Note that
+.B ldconfig
+will only look at files that are named
+.I lib*.so*
+(for regular shared objects) or
+.I ld-*.so*
+(for the dynamic loader itsef).  Other files will be ignored.  Also,
+.B ldconfig
+expects a certain pattern to how the symlinks are set up, like this
+example, where the middle file
+.RB ( libfoo.so.1
+here) is the SONAME for the library:
+.PP
+.in +4n
+.EX
+libfoo.so -> libfoo.so.1 -> libfoo.so.1.12
+.EE
+.in
+.PP
+Failure to follow this pattern may result in compatibility issues
+after an upgrade.
 .SH OPTIONS
 .TP
 .BR \-c " \fIfmt\fP, " \-\-format=\fIfmt\fP