[DTrace-devel] [External] : [PATCH 4/7] usdt: place uprobes by file offset

[Eslam Almorshdy via B4 Relay]

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

dt_pid_create_usdt_probes_proc() converts each tracepoint address from
dtprobed to a uprobe offset by subtracting the load address of the
object's first segment.  That gives the address's distance from the
first segment, not its file offset: the same mistake as in the pid
provider, with the same effect on lld-linked executables and libraries.
Every USDT and is-enabled probe is placed 0x1000 bytes past its site.
In a small object, that address lies outside any executable mapping, so
the probe never fires and is-enabled stays false; otherwise the probe
fires in other code or makes enabling fail for the whole process
(ENOTSUPP).

Convert through the mapping that contains the address instead.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Eslam Almorshdy <ealmorshdy at d-matrix.ai>
---
 libdtrace/dt_pid.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libdtrace/dt_pid.c b/libdtrace/dt_pid.c
index 582862e4..7741e6d9 100644
--- a/libdtrace/dt_pid.c
+++ b/libdtrace/dt_pid.c
@@ -1087,7 +1087,14 @@ dt_pid_create_usdt_probes_proc(dtrace_hdl_t *dtp, pid_t pid, dt_proc_t *dpr,
 			psp.pps_dev = pmp->pr_dev;
 			psp.pps_inum = pmp->pr_inum;
 			psp.pps_pid = dpr->dpr_pid;
-			psp.pps_off = tp->tracepoint.addr - pmp->pr_file->first_segment->pr_vaddr;
+			/*
+			 * uprobes are placed by file offset: convert through
+			 * the mapping that contains the tracepoint.  That need
+			 * not be the first segment, and its p_vaddr - p_offset
+			 * can differ from the first segment's (LLVM lld).
+			 */
+			psp.pps_off = tp->tracepoint.addr - pmp->pr_vaddr +
+				      pmp->pr_offset;
 			psp.pps_nameoff = 0;
 
 			if (nargv) {

-- 
2.55.0