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target/arm: Avoid shifts by -1 in tszimm_shr() and tszimm_shl()

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The function tszimm_esz() returns a shift amount, or possibly -1 in
certain cases that correspond to unallocated encodings in the
instruction set.  We catch these later in the trans_ functions
(generally with an "a-esz < 0" check), but before we do the
decodetree-generated code will also call tszimm_shr() or tszimm_sl(),
which will use the tszimm_esz() return value as a shift count without
checking that it is not negative, which is undefined behaviour.

Avoid the UB by checking the return value in tszimm_shr() and
tszimm_shl().

Cc: qemu-stable@nongnu.org
Resolves: Coverity CID 1547617, 1547694
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240722172957.1041231-4-peter.maydell@linaro.org
This commit is contained in:
Peter Maydell 2024-07-22 18:29:56 +01:00
parent ea3f5a90f0
commit 76916dfa89
1 changed files with 16 additions and 2 deletions
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18
target/arm/tcg/translate-sve.c
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@ -50,13 +50,27 @@ static int tszimm_esz(DisasContext *s, int x)
static int tszimm_shr(DisasContext *s, int x)
{
return (16 << tszimm_esz(s, x)) - x;
/*
* We won't use the tszimm_shr() value if tszimm_esz() returns -1 (the
* trans function will check for esz < 0), so we can return any
* value we like from here in that case as long as we avoid UB.
*/
int esz = tszimm_esz(s, x);
if (esz < 0) {
return esz;
}
return (16 << esz) - x;
}
/* See e.g. LSL (immediate, predicated). */
static int tszimm_shl(DisasContext *s, int x)
{
return x - (8 << tszimm_esz(s, x));
/* As with tszimm_shr(), value will be unused if esz < 0 */
int esz = tszimm_esz(s, x);
if (esz < 0) {
return esz;
}
return x - (8 << esz);
}
/* The SH bit is in bit 8. Extract the low 8 and shift. */