[PATCH] perf build: fix in-tree build

[Luca Ceresoli]

Building perf in-tree is broken after commit 890a1961c812 ("perf tools:
Create source symlink in perf object dir") which added a 'source' symlink
in the output dir pointing to the source dir.

With in-tree builds, the added 'SOURCE = ...' line is executed multiple
times (I observed 2 during the build plus 2 during installation). This is a
minor inefficiency, in theory not harmful because symlink creation is
assumed to be idempotent. But it is not.

Considering with in-tree builds:

  srctree=/absolute/path/to/linux
   OUTPUT=/absolute/path/to/linux/tools/perf

here's what happens:

 1. ln -sf $(srctree)/tools/perf $(OUTPUT)/source
    -> creates /absolute/path/to/linux/tools/perf/source
       link to /absolute/path/to/linux/tools/perf
    => OK, that's what was intended
 2. ln -sf $(srctree)/tools/perf $(OUTPUT)/source   # same command as 1
    -> creates /absolute/path/to/linux/tools/perf/perf
       link to /absolute/path/to/linux/tools/perf
    => Not what was intended, not idempotent
 3. Now the build _should_ create the 'perf' executable, but it fails

The reason is the tricky 'ln' command line. At the first invocation 'ln'
uses the 1st form:

       ln [OPTION]... [-T] TARGET LINK_NAME

and creates a link to TARGET *called LINK_NAME*.

At the second invocation $(OUTPUT)/source exists, so 'ln' uses the 3rd
form:

       ln [OPTION]... TARGET... DIRECTORY

and creates a link to TARGET *called TARGET* inside DIRECTORY.

Fix by adding --no-dereference to "treat LINK_NAME as a normal file if it
is a symbolic link to a directory", as the manpage says.

Link: https://lore.kernel.org/all/20241125182506.38af9907@booty/
Fixes: 890a1961c812 ("perf tools: Create source symlink in perf object dir")
Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
---
 tools/perf/Makefile.perf | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index d74241a151313bd09101aabb5d765a5a0a6efc84..bbd799a0fd544db220f29d1e250a819a765d04f3 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -164,7 +164,7 @@ ifneq ($(OUTPUT),)
 VPATH += $(OUTPUT)
 export VPATH
 # create symlink to the original source
-SOURCE := $(shell ln -sf $(srctree)/tools/perf $(OUTPUT)/source)
+SOURCE := $(shell ln -sf --no-dereference $(srctree)/tools/perf $(OUTPUT)/source)
 endif
 
 ifeq ($(V),1)

---
base-commit: ffd294d346d185b70e28b1a28abe367bbfe53c04
change-id: 20250124-perf-fix-intree-build-fbd97f560254

Best regards,
-- 
Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>