Merge branch 'bpf-introduce-file-dynptr'

Mykyta Yatsenko says:

====================
bpf: Introduce file dynptr

From: Mykyta Yatsenko <yatsenko@meta.com>

This series adds a new dynptr kind, file dynptr, which enables BPF
programs to perform safe reads from files in a structured way.
Initial motivations include:
 * Parsing the executable’s ELF to locate thread-local variable symbols
 * Capturing stack traces when frame pointers are disabled

By leveraging the existing dynptr abstraction, we reuse the verifier’s
lifetime/size checks and keep the API consistent with existing dynptr
read helpers.

Technical details:
1. Reuses the existing freader library to read files a folio at a time.
2. bpf_dynptr_slice() and bpf_dynptr_read() always copy data from folios
into a program-provided buffer; zero-copy access is intentionally not
supported to keep it simple.
3. Reads may sleep if the requested folios are not in the page cache.
4. Few verifier changes required:
  * Support dynptr destruction in kfuncs
  * Add kfunc address substitution based on whether the program runs in
  a sleepable or non-sleepable context.

Testing:
The final patch adds a selftest that validates BPF program reads the
same data as userspace, page faults are enabled in sleepable context and
disabled in non-sleepable.

Changelog:
---
v4 -> v5
v4: https://lore.kernel.org/all/20251021200334.220542-1-mykyta.yatsenko5@gmail.com/
 * Inlined and removed kfunc_call_imm(), run overflow check for call_imm
 only if !bpf_jit_supports_far_kfunc_call().

v3 -> v4
v3: https://lore.kernel.org/bpf/20251020222538.932915-1-mykyta.yatsenko5@gmail.com/
 * Remove ringbuf usage from selftests
 * bpf_dynptr_set_null(ptr) when discarding file dynptr
 * call kfunc_call_imm() in specialize_kfunc() only, removed
 call from add_kfunc_call()

v2 -> v3
v2: https://lore.kernel.org/bpf/20251015161155.120148-1-mykyta.yatsenko5@gmail.com/
 * Add negative tests
 * Rewrote tests to use LSM for bpf_get_task_exe_file()
 * Move call_imm overflow check into kfunc_call_imm()

v1 -> v2
v1: https://lore.kernel.org/bpf/20251003160416.585080-1-mykyta.yatsenko5@gmail.com/
 * Remove ELF parsing selftest
 * Expanded u32 -> u64 refactoring, changes in include/uapi/linux/bpf.h
 * Removed freader.{c,h}, instead move freader definitions into
 buildid.h.
 * Small refactoring of the multiple folios reading algorithm
 * Directly return error after unmark_stack_slots_dynptr().
 * Make kfuncs receive trusted arguments.
 * Remove enum bpf_is_sleepable, use bool instead
 * Remove unnecessary sorting from specialize_kfunc()
 * Remove bool kfunc_in_sleepable_ctx; field from the struct
 bpf_insn_aux_data, rely on non_sleepable field introduced by Kumar
 * Refactor selftests, do madvise(...MADV_PAGEOUT) for all pages read by
 the test
 * Introduce the test for non-sleepable case, verify it fails with -EFAULT
====================

Link: https://lore.kernel.org/r/20251026203853.135105-1-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 20 insertions, 36 deletions

diff --git a/lib/buildid.c b/lib/buildid.c
index c4b0f376fb34..aaf61dfc0919 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -11,27 +11,8 @@
 
 #define MAX_PHDR_CNT 256
 
-struct freader {
-	void *buf;
-	u32 buf_sz;
-	int err;
-	union {
-		struct {
-			struct file *file;
-			struct folio *folio;
-			void *addr;
-			loff_t folio_off;
-			bool may_fault;
-		};
-		struct {
-			const char *data;
-			u64 data_sz;
-		};
-	};
-};
-
-static void freader_init_from_file(struct freader *r, void *buf, u32 buf_sz,
-				   struct file *file, bool may_fault)
+void freader_init_from_file(struct freader *r, void *buf, u32 buf_sz,
+			    struct file *file, bool may_fault)
 {
 	memset(r, 0, sizeof(*r));
 	r->buf = buf;
@@ -40,7 +21,7 @@ static void freader_init_from_file(struct freader *r, void *buf, u32 buf_sz,
 	r->may_fault = may_fault;
 }
 
-static void freader_init_from_mem(struct freader *r, const char *data, u64 data_sz)
+void freader_init_from_mem(struct freader *r, const char *data, u64 data_sz)
 {
 	memset(r, 0, sizeof(*r));
 	r->data = data;
@@ -92,7 +73,7 @@ static int freader_get_folio(struct freader *r, loff_t file_off)
 	return 0;
 }
 
-static const void *freader_fetch(struct freader *r, loff_t file_off, size_t sz)
+const void *freader_fetch(struct freader *r, loff_t file_off, size_t sz)
 {
 	size_t folio_sz;
 
@@ -127,18 +108,21 @@ static const void *freader_fetch(struct freader *r, loff_t file_off, size_t sz)
 	 */
 	folio_sz = folio_size(r->folio);
 	if (file_off + sz > r->folio_off + folio_sz) {
-		int part_sz = r->folio_off + folio_sz - file_off;
-
-		/* copy the part that resides in the current folio */
-		memcpy(r->buf, r->addr + (file_off - r->folio_off), part_sz);
-
-		/* fetch next folio */
-		r->err = freader_get_folio(r, r->folio_off + folio_sz);
-		if (r->err)
-			return NULL;
-
-		/* copy the rest of requested data */
-		memcpy(r->buf + part_sz, r->addr, sz - part_sz);
+		u64 part_sz = r->folio_off + folio_sz - file_off, off;
+
+		memcpy(r->buf, r->addr + file_off - r->folio_off, part_sz);
+		off = part_sz;
+
+		while (off < sz) {
+			/* fetch next folio */
+			r->err = freader_get_folio(r, r->folio_off + folio_sz);
+			if (r->err)
+				return NULL;
+			folio_sz = folio_size(r->folio);
+			part_sz = min_t(u64, sz - off, folio_sz);
+			memcpy(r->buf + off, r->addr, part_sz);
+			off += part_sz;
+		}
 
 		return r->buf;
 	}
@@ -147,7 +131,7 @@ static const void *freader_fetch(struct freader *r, loff_t file_off, size_t sz)
 	return r->addr + (file_off - r->folio_off);
 }
 
-static void freader_cleanup(struct freader *r)
+void freader_cleanup(struct freader *r)
 {
 	if (!r->buf)
 		return; /* non-file-backed mode */