| Message ID | 20230517181353.381073-1-kursad.oney@broadcom.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | ARM: memset: cast the constant byte to unsigned char | expand |
Hi Ard, On Wed, May 17, 2023 at 2:14 PM Kursad Oney <kursad.oney@broadcom.com> wrote: > > memset() description in ISO/IEC 9899:1999 (and elsewhere) says: > > The memset function copies the value of c (converted to an > unsigned char) into each of the first n characters of the > object pointed to by s. > > The kernel's arm32 memset does not cast c to unsigned char. This results > in the following code to produce erroneous output: > > char a[128]; > memset(a, -128, sizeof(a)); > > This is because gcc will generally emit the following code before > it calls memset() : > > mov r0, r7 > mvn r1, #127 ; 0x7f > bl 00000000 <memset> > > r1 ends up with 0xffffff80 before being used by memset() and the > 'a' array will have -128 once in every four bytes while the other > bytes will be set incorrectly to -1 like this (printing the first > 8 bytes) : > > test_module: -128 -1 -1 -1 > test_module: -1 -1 -1 -128 > > The change here is to 'and' r1 with 255 before it is used. > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > Signed-off-by: Kursad Oney <kursad.oney@broadcom.com> > > --- > > arch/arm/lib/memset.S | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/arm/lib/memset.S b/arch/arm/lib/memset.S > index d71ab61430b2..de75ae4d5ab4 100644 > --- a/arch/arm/lib/memset.S > +++ b/arch/arm/lib/memset.S > @@ -17,6 +17,7 @@ ENTRY(__memset) > ENTRY(mmioset) > WEAK(memset) > UNWIND( .fnstart ) > + and r1, r1, #255 @ cast to unsigned char > ands r3, r0, #3 @ 1 unaligned? > mov ip, r0 @ preserve r0 as return value > bne 6f @ 1 > -- > 2.37.3 > I didn't get any reaction to this patch so I added you to see if you could help review it or direct me to the right channel. Thank you! kursad
On Wed, 17 May 2023 at 20:14, Kursad Oney <kursad.oney@broadcom.com> wrote: > > memset() description in ISO/IEC 9899:1999 (and elsewhere) says: > > The memset function copies the value of c (converted to an > unsigned char) into each of the first n characters of the > object pointed to by s. > > The kernel's arm32 memset does not cast c to unsigned char. This results > in the following code to produce erroneous output: > > char a[128]; > memset(a, -128, sizeof(a)); > > This is because gcc will generally emit the following code before > it calls memset() : > > mov r0, r7 > mvn r1, #127 ; 0x7f > bl 00000000 <memset> > > r1 ends up with 0xffffff80 before being used by memset() and the > 'a' array will have -128 once in every four bytes while the other > bytes will be set incorrectly to -1 like this (printing the first > 8 bytes) : > > test_module: -128 -1 -1 -1 > test_module: -1 -1 -1 -128 > > The change here is to 'and' r1 with 255 before it is used. > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > Signed-off-by: Kursad Oney <kursad.oney@broadcom.com> > > --- > > arch/arm/lib/memset.S | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/arm/lib/memset.S b/arch/arm/lib/memset.S > index d71ab61430b2..de75ae4d5ab4 100644 > --- a/arch/arm/lib/memset.S > +++ b/arch/arm/lib/memset.S > @@ -17,6 +17,7 @@ ENTRY(__memset) > ENTRY(mmioset) > WEAK(memset) > UNWIND( .fnstart ) > + and r1, r1, #255 @ cast to unsigned char > ands r3, r0, #3 @ 1 unaligned? > mov ip, r0 @ preserve r0 as return value > bne 6f @ 1 Yes, this is clearly a bug. The value in R1 is expanded to 32 bits like this 1: orr r1, r1, r1, lsl #8 orr r1, r1, r1, lsl #16 which assumes that the upper bytes are 0x0, which they are not in this case. Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
On Wed, May 17, 2023 at 8:14 PM Kursad Oney <kursad.oney@broadcom.com> wrote: > memset() description in ISO/IEC 9899:1999 (and elsewhere) says: > > The memset function copies the value of c (converted to an > unsigned char) into each of the first n characters of the > object pointed to by s. > > The kernel's arm32 memset does not cast c to unsigned char. This results > in the following code to produce erroneous output: > > char a[128]; > memset(a, -128, sizeof(a)); > > This is because gcc will generally emit the following code before > it calls memset() : > > mov r0, r7 > mvn r1, #127 ; 0x7f > bl 00000000 <memset> > > r1 ends up with 0xffffff80 before being used by memset() and the > 'a' array will have -128 once in every four bytes while the other > bytes will be set incorrectly to -1 like this (printing the first > 8 bytes) : > > test_module: -128 -1 -1 -1 > test_module: -1 -1 -1 -128 > > The change here is to 'and' r1 with 255 before it is used. > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > Signed-off-by: Kursad Oney <kursad.oney@broadcom.com> Wow you found this old thing! Reviewed-by: Linus Walleij <linus.walleij@linaro.org> Can you please put this into Russell's patch tracker? https://www.arm.linux.org.uk/developer/ Yours, Linus Walleij
Hi Linus, Ard, On Thu, Aug 10, 2023 at 4:15 AM Linus Walleij <linus.walleij@linaro.org> wrote: > > On Wed, May 17, 2023 at 8:14 PM Kursad Oney <kursad.oney@broadcom.com> wrote: > > > memset() description in ISO/IEC 9899:1999 (and elsewhere) says: > > > > The memset function copies the value of c (converted to an > > unsigned char) into each of the first n characters of the > > object pointed to by s. > > > > The kernel's arm32 memset does not cast c to unsigned char. This results > > in the following code to produce erroneous output: > > > > char a[128]; > > memset(a, -128, sizeof(a)); > > > > This is because gcc will generally emit the following code before > > it calls memset() : > > > > mov r0, r7 > > mvn r1, #127 ; 0x7f > > bl 00000000 <memset> > > > > r1 ends up with 0xffffff80 before being used by memset() and the > > 'a' array will have -128 once in every four bytes while the other > > bytes will be set incorrectly to -1 like this (printing the first > > 8 bytes) : > > > > test_module: -128 -1 -1 -1 > > test_module: -1 -1 -1 -128 > > > > The change here is to 'and' r1 with 255 before it is used. > > > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") > > Signed-off-by: Kursad Oney <kursad.oney@broadcom.com> > > Wow you found this old thing! > Reviewed-by: Linus Walleij <linus.walleij@linaro.org> > > Can you please put this into Russell's patch tracker? > https://www.arm.linux.org.uk/developer/ Thank you for the reviews. I just got back from vacation and will add this into Russell's tracker. > > Yours, > Linus Walleij
diff --git a/arch/arm/lib/memset.S b/arch/arm/lib/memset.S index d71ab61430b2..de75ae4d5ab4 100644 --- a/arch/arm/lib/memset.S +++ b/arch/arm/lib/memset.S @@ -17,6 +17,7 @@ ENTRY(__memset) ENTRY(mmioset) WEAK(memset) UNWIND( .fnstart ) + and r1, r1, #255 @ cast to unsigned char ands r3, r0, #3 @ 1 unaligned? mov ip, r0 @ preserve r0 as return value bne 6f @ 1
memset() description in ISO/IEC 9899:1999 (and elsewhere) says: The memset function copies the value of c (converted to an unsigned char) into each of the first n characters of the object pointed to by s. The kernel's arm32 memset does not cast c to unsigned char. This results in the following code to produce erroneous output: char a[128]; memset(a, -128, sizeof(a)); This is because gcc will generally emit the following code before it calls memset() : mov r0, r7 mvn r1, #127 ; 0x7f bl 00000000 <memset> r1 ends up with 0xffffff80 before being used by memset() and the 'a' array will have -128 once in every four bytes while the other bytes will be set incorrectly to -1 like this (printing the first 8 bytes) : test_module: -128 -1 -1 -1 test_module: -1 -1 -1 -128 The change here is to 'and' r1 with 255 before it is used. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Kursad Oney <kursad.oney@broadcom.com> --- arch/arm/lib/memset.S | 1 + 1 file changed, 1 insertion(+)