[DTrace-devel] [External] : [PATCH 6/7] dtrace provider: place uprobes by file offset

[Eslam Almorshdy via B4 Relay]

From: Eslam Almorshdy <ealmorshdy at d-matrix.ai>

uprobe_spec() places the dtrace provider's BEGIN, END and ERROR uprobes
on the *_probe() functions in libdtrace.so at the symbol's distance from
the object's first segment, not at its file offset: the same mistake as
in the pid and USDT providers.  With a libdtrace linked by lld,
dtrace:::BEGIN is placed 0x1000 bytes past BEGIN_probe(), never fires,
and every session hangs.

Convert through the mapping that contains the symbol instead.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Eslam Almorshdy <ealmorshdy at d-matrix.ai>
---
 libdtrace/dt_prov_dtrace.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/libdtrace/dt_prov_dtrace.c b/libdtrace/dt_prov_dtrace.c
index 4b788507..b8437c17 100644
--- a/libdtrace/dt_prov_dtrace.c
+++ b/libdtrace/dt_prov_dtrace.c
@@ -207,11 +207,13 @@ static char *uprobe_spec(pid_t pid, const char *prb)
 		if (mapp == NULL)
 			goto out;
 
-		if (mapp->pr_file->first_segment != mapp)
-			mapp = mapp->pr_file->first_segment;
-
+		/*
+		 * The uprobe spec takes a file offset: convert through the
+		 * mapping that contains the symbol, not the first segment
+		 * (their p_vaddr - p_offset differ, e.g. with LLVM lld).
+		 */
 		asprintf(&spec, "%s:0x%lx", mapp->pr_file->prf_mapname,
-			 sym.st_value - mapp->pr_vaddr);
+			 sym.st_value - mapp->pr_vaddr + mapp->pr_offset);
 	}
 
 out:

-- 
2.55.0