[PATCH 1/2] profile: account for partial /proc/profile reads

[PATCH 1/2] profile: account for partial /proc/profile reads

[Andrew Morton]

From: Andrew Morton <akpm@linux-foundation.org>
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@linux-foundation.org>
---
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 */
_

[PATCH 2/2] profile: simplify /proc/profile read layout

[Andrew Morton]

From: Andrew Morton <akpm@linux-foundation.org>
Subject: profile: simplify /proc/profile read layout
Date: Tue, 20 Oct 2026 23:58:33 +0000

read_profile() is needlessly difficult to understand.  It copies the
four-byte sampling-step header one byte at a time, then manufactures
a pointer before prof_buffer so a file offset can be used as a
counter-array offset.  Its signed byte counter, unsigned long file
offset and repeated size calculations further obscure where each
read starts and how many bytes it transfers.

Treat the file as two consecutive regions: a four-byte header and
the profiling counters.  Use size_t for byte positions and lengths,
copy the bounded header in one operation, and address the counters
relative to the start of prof_buffer.  Brief comments explain each
region.

Keep the word-sized shift and explicit narrowing required by Avi
Weiss's "profiling: fix sampling step on 64-bit big-endian systems".
Assign the header value separately and explain why the 32-bit ABI
requires a narrowing conversion.

Preserve the partial-read and fault semantics established by [1/2],
including short copies within either region.  No userspace-visible
behavior change is intended.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
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/unaligned and boundary reads, EOF, partial copy faults,
return values and file-position updates.
The 64-bit big-endian case was not tested. No per-patch or
unpatched-versus-patched runtime comparison was reported.

 kernel/profile.c |   57 +++++++++++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 22 deletions(-)

--- a/kernel/profile.c~profile-simplify-proc-profile-read-layout
+++ a/kernel/profile.c
@@ -153,35 +153,48 @@ void profile_tick(int type)
 static ssize_t
 read_profile(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 {
-	unsigned long p = *ppos;
-	ssize_t read;
-	char *pnt;
-	unsigned int sample_step = (unsigned int)(1UL << prof_shift);
-	size_t not_copied;
+	const size_t header_size = sizeof(unsigned int);
+	const size_t file_size = (prof_len + 1) * sizeof(unsigned int);
+	unsigned int sample_step;
+	size_t pos = *ppos;
+	size_t done = 0;
+	size_t n, not_copied;
 
-	if (p >= (prof_len+1)*sizeof(unsigned int))
+	/* Keep the shift word-sized before narrowing to the 32-bit ABI. */
+	sample_step = (unsigned int)(1UL << prof_shift);
+
+	if (pos >= file_size)
 		return 0;
-	if (count > (prof_len+1)*sizeof(unsigned int) - p)
-		count = (prof_len+1)*sizeof(unsigned int) - p;
-	read = 0;
+	count = min(count, file_size - pos);
 
-	while (p < sizeof(unsigned int) && count > 0) {
-		if (put_user(*((char *)(&sample_step)+p), buf))
-			goto out;
-		buf++; p++; count--; read++;
+	/* Copy the four-byte sampling resolution. */
+	if (pos < header_size) {
+		n = min(count, header_size - pos);
+		not_copied = copy_to_user(buf, (u8 *)&sample_step + pos, n);
+		done += n - not_copied;
+		if (not_copied)
+			goto fault;
+		buf += n;
+		pos += n;
+		count -= n;
 	}
+
+	/* Copy raw counters using offsets relative to the counter array. */
 	if (count) {
-		pnt = (char *)prof_buffer + p - sizeof(atomic_t);
-		not_copied = copy_to_user(buf, pnt, count);
-		read += count - not_copied;
+		not_copied = copy_to_user(buf,
+					(u8 *)prof_buffer + pos - header_size,
+					count);
+		done += count - not_copied;
 		if (not_copied)
-			goto out;
+			goto fault;
 	}
-	*ppos += read;
-	return read;
-out:
-	*ppos += read;
-	return read ? read : -EFAULT;
+	*ppos += done;
+	return done;
+fault:
+	*ppos += done;
+	if (!done)
+		return -EFAULT;
+	return done;
 }
 
 /* default is to not implement this call */
_

Re: [PATCH 1/2] profile: account for partial /proc/profile reads

[Andrew Morton]

On Tue, 20 Oct 2026 17:47:06 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> 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.

Sashiko: "Failed To Apply".

bah.  I'll resend all three.