Re: [BUG] bpf: tcp: bpf_tcp_ops_set_rcvlowat() missing from BTF with clang KCSAN

[Kuniyuki Iwashima <kuniyu@google.com>]

On Fri, 06 Nov 2026 07:58:01 -0800, Breno Leitao wrote:
> On Fri, Nov 06, 2026 at 09:16:24PM +0800, Jiayuan Chen wrote:
> > Thanks for the report !
> > 
> > 
> > On 11/6/26 7:38 PM, Breno Leitao wrote:
> > > My arm64 KCSAN builds of linux-next fail since next-20261015, because
> > > bpf_tcp_ops_set_rcvlowat() is missing from the vmlinux BTF:
> > >
> > >   WARN: resolve_btfids: no BTF func for kfunc bpf_tcp_ops_set_rcvlowat in bpf_tcp_ops_set_rcvlowat_kfunc_set
> > >   WARN: resolve_btfids: unresolved symbol bpf_tcp_ops_set_rcvlowat
> > >   Failed to generate BTF for vmlinux
> > >   Try to disable CONFIG_DEBUG_INFO_BTF
> > >   make[4]: *** [scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 1
> > 
> > 
> > 
> > Has pahole fixed this issue?
> 
> I don't think so, see the line below:

Could you try this ?

---8<---
diff --git a/dwarf_loader.c b/dwarf_loader.c
index 01049e9802a3..92793ae6cf98 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1782,18 +1782,32 @@ static uint64_t dwarf_op__const_value(const Dwarf_Op *op)
 	return op->number;
 }
 
+static bool cu__is_arg_reg(const struct cu *cu, int reg)
+{
+	if (cu->register_params[0] < 0)
+		return reg >= DW_OP_reg0;
+
+	for (int i = 0; i < cu->nr_register_params; i++) {
+		if (cu->register_params[i] == reg)
+			return true;
+	}
+	return false;
+}
+
 static void parameter__set_loc_reg(struct parameter *parm, int reg)
 {
 	if (parm->loc_reg == PARAMETER_UNKNOWN_REG)
 		parm->loc_reg = reg;
 }
 
-static void parameter__record_loc_reg(struct parameter *parm, int reg,
-				      int64_t offset, bool is_deref,
-				      uint8_t size)
+static void parameter__record_loc_reg(struct parameter *parm, const struct cu *cu,
+				      bool for_inline, int reg, int64_t offset,
+				      bool is_deref, uint8_t size)
 {
 	if (reg < DW_OP_reg0 || !size)
 		return;
+	if (!for_inline && (is_deref || !cu__is_arg_reg(cu, reg)))
+		return;
 
 	parameter__set_loc_reg(parm, reg);
 
@@ -1920,7 +1934,8 @@ static void parameter__finish_piece_decode(struct parameter *parm, Dwarf_Die *di
  * ABI-preserved or should be replaced by the single used member candidate.
  */
 static void parameter__multi_exprs(Dwarf_Op *expr, int loc_num, struct cu *cu,
-				   size_t exprlen, struct parameter *parm)
+				   size_t exprlen, struct parameter *parm,
+				   bool for_inline)
 {
 	int regs[2], nregs = 0;
 	uint64_t piece_size = 0;
@@ -1951,11 +1966,12 @@ static void parameter__multi_exprs(Dwarf_Op *expr, int loc_num, struct cu *cu,
 		switch (expr[0].atom) {
 		case DW_OP_reg0 ... DW_OP_reg31:
 			if (loc_num == 0)
-				parameter__record_loc_reg(parm, expr[0].atom, 0, false, size);
+				parameter__record_loc_reg(parm, cu, for_inline, expr[0].atom,
+							  0, false, size);
 			return;
 		case DW_OP_breg0 ... DW_OP_breg31:
 			if (loc_num == 0)
-				parameter__record_loc_reg(parm,
+				parameter__record_loc_reg(parm, cu, for_inline,
 							  expr[0].atom - DW_OP_breg0 + DW_OP_reg0,
 							  dwarf_op__snumber(&expr[0]),
 							  !had_stack_value, size);
@@ -1994,14 +2010,15 @@ static void parameter__multi_exprs(Dwarf_Op *expr, int loc_num, struct cu *cu,
 			if (off < cu->addr_size || parm->loc_reg == PARAMETER_UNKNOWN_REG) {
 				uint8_t size = parameter__loc_size(parm, cu);
 
-				parameter__record_loc_reg(parm, expr[i].atom, 0, false, size);
+				parameter__record_loc_reg(parm, cu, for_inline, expr[i].atom,
+							  0, false, size);
 			}
 		} else {
 			reg_pieces = false;
 		}
 	}
 
-	if (loc_num == 0 && reg_pieces && nregs == ARRAY_SIZE(regs) &&
+	if (for_inline && loc_num == 0 && reg_pieces && nregs == ARRAY_SIZE(regs) &&
 	    piece_size == parm->type_byte_size)
 		parameter__record_loc_reg_pair(parm, DW_OP_reg0 + regs[0],
 					       DW_OP_reg0 + regs[1]);
@@ -2009,7 +2026,7 @@ static void parameter__multi_exprs(Dwarf_Op *expr, int loc_num, struct cu *cu,
 
 static void parameter__decode_location(Dwarf_Attribute *attr, struct conf_load *conf,
 				       struct cu *cu, Dwarf_Die *die,
-				       struct parameter *parm)
+				       struct parameter *parm, bool for_inline)
 {
 	Dwarf_Addr base, start, end;
 	Dwarf_Op *expr, *entry_ops;
@@ -2051,7 +2068,7 @@ static void parameter__decode_location(Dwarf_Attribute *attr, struct conf_load *
 			continue;
 
 		if (exprlen != 1) {
-			parameter__multi_exprs(expr, loc_num, cu, exprlen, parm);
+			parameter__multi_exprs(expr, loc_num, cu, exprlen, parm, for_inline);
 			continue;
 		}
 
@@ -2060,11 +2077,12 @@ static void parameter__decode_location(Dwarf_Attribute *attr, struct conf_load *
 		switch (expr->atom) {
 		case DW_OP_reg0 ... DW_OP_reg31:
 			if (loc_num == 0)
-				parameter__record_loc_reg(parm, expr->atom, 0, false, size);
+				parameter__record_loc_reg(parm, cu, for_inline, expr->atom,
+							  0, false, size);
 			break;
 		case DW_OP_breg0 ... DW_OP_breg31:
 			if (loc_num == 0)
-				parameter__record_loc_reg(parm,
+				parameter__record_loc_reg(parm, cu, for_inline,
 							  expr->atom - DW_OP_breg0 + DW_OP_reg0,
 							  dwarf_op__snumber(expr),
 							  had_deref || !had_stack_value,
@@ -2103,7 +2121,8 @@ static void parameter__decode_location(Dwarf_Attribute *attr, struct conf_load *
 			if (dwarf_getlocation_attr(attr, expr, &entry_attr) == 0 &&
 			    dwarf_getlocation(&entry_attr, &entry_ops, &entry_len) == 0 &&
 			    entry_len == 1 && dwarf_op__is_reg(entry_ops->atom))
-				parameter__record_loc_reg(parm, entry_ops->atom, 0, false, size);
+				parameter__record_loc_reg(parm, cu, for_inline, entry_ops->atom,
+							  0, false, size);
 			break;
 		}
 	}
@@ -2171,7 +2190,8 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu, struct co
 			parameter__record_attr_const_value(&attr, cu, parm);
 		parm->has_loc = dwarf_attr(die, DW_AT_location, &attr) != NULL;
 		if (parm->has_loc)
-			parameter__decode_location(&attr, conf, cu, die, parm);
+			parameter__decode_location(&attr, conf, cu, die, parm,
+						   ftype == NULL);
 	}
 
 	return parm;
---8<---