[PATCH 2/3] profile: account for partial /proc/profile reads

[Andrew Morton]

From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Subject: profile: account for partial /proc/profile reads
Date: Tue, 20 Oct 2026 15:28:00 -0700

read_profile() returns -EFAULT without advancing the file position
when a user copy faults, even if earlier header bytes or a prefix of
the counters were successfully copied.  A retry can therefore copy
the same bytes again despite the previous read having delivered data.

Return the number of successfully copied bytes and advance *ppos by
that amount.  Return -EFAULT only if the fault occurs before any bytes
are copied.  Preserve the four-byte sampling-step header, raw counter
representation, and normal successful-read behavior.

Avi Weiss's "profiling: fix sampling step on 64-bit big-endian
systems" is unchanged.  This patch changes only short-read and
fault reporting, not the output format.
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Avi Weiss <thnkslprpt@xxxxxxxxx>
Cc: Bradley Morgan <brads@xxxxxxxxxxxxxx>
---
Tested the combined three-patch series on x86-64 QEMU/KVM with
CONFIG_PROFILING=y, CONFIG_PROC_FS=y and profile=12.
profile-read-test.c: PASS (0 failures). Covered header decoding,
short and boundary reads, EOF, partial user-copy faults, return
values and file-position accounting.
The 64-bit big-endian fix was not runtime tested. There was no
individual-patch or unpatched-versus-patched runtime comparison.

 kernel/profile.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

--- a/kernel/profile.c~profile-account-for-partial-proc-profile-reads
+++ a/kernel/profile.c
@@ -157,6 +157,7 @@ read_profile(struct file *file, char __u
 	ssize_t read;
 	char *pnt;
 	unsigned int sample_step = (unsigned int)(1UL << prof_shift);
+	size_t not_copied;
 
 	if (p >= (prof_len+1)*sizeof(unsigned int))
 		return 0;
@@ -166,15 +167,21 @@ read_profile(struct file *file, char __u
 
 	while (p < sizeof(unsigned int) && count > 0) {
 		if (put_user(*((char *)(&sample_step)+p), buf))
-			return -EFAULT;
+			goto out;
 		buf++; p++; count--; read++;
 	}
-	pnt = (char *)prof_buffer + p - sizeof(atomic_t);
-	if (copy_to_user(buf, (void *)pnt, count))
-		return -EFAULT;
-	read += count;
+	if (count) {
+		pnt = (char *)prof_buffer + p - sizeof(atomic_t);
+		not_copied = copy_to_user(buf, pnt, count);
+		read += count - not_copied;
+		if (not_copied)
+			goto out;
+	}
 	*ppos += read;
 	return read;
+out:
+	*ppos += read;
+	return read ? read : -EFAULT;
 }
 
 /* default is to not implement this call */
_